[compiler] Add fallthrough to branch terminal

Branch terminals didn't have a fallthrough because they correspond to an outer terminal (optional, logical, etc) that has the "real" fallthrough. But understanding how branch terminals correspond to these outer terminals requires knowing the branch fallthrough. For example, `foo?.bar?.baz` creates terminals along the lines of:

```
bb0:
  optional fallthrough=bb4
bb1:
  optional fallthrough=bb3
bb2:
  ...
  branch ... (fallthrough=bb3)

...

bb3:
  ...
  branch ... (fallthrough=bb4)

...

bb4:
  ...
```

Without a fallthrough on `branch` terminals, it's unclear that the optional from bb0 has its branch node in bb3. With the fallthroughs, we can see look for a branch with the same fallthrough as the outer optional terminal to match them up.

[ghstack-poisoned]

compiler/packages/babel-plugin-react-compiler/src/HIR/BuildHIR.ts

@@ -607,6 +607,7 @@ function lowerStatement(
             ),
             consequent: bodyBlock,
             alternate: continuationBlock.id,
+            fallthrough: continuationBlock.id,
             id: makeInstructionId(0),
             loc: stmt.node.loc ?? GeneratedSource,
           },
@@ -656,16 +657,13 @@ function lowerStatement(
         },
         conditionalBlock,
       );
-      /*
-       * The conditional block is empty and exists solely as conditional for
-       * (re)entering or exiting the loop
-       */
       const test = lowerExpressionToTemporary(builder, stmt.get('test'));
       const terminal: BranchTerminal = {
         kind: 'branch',
         test,
         consequent: loopBlock,
         alternate: continuationBlock.id,
+        fallthrough: conditionalBlock.id,
         id: makeInstructionId(0),
         loc: stmt.node.loc ?? GeneratedSource,
       };
@@ -975,6 +973,7 @@ function lowerStatement(
         test,
         consequent: loopBlock,
         alternate: continuationBlock.id,
+        fallthrough: conditionalBlock.id,
         id: makeInstructionId(0),
         loc,
       };
@@ -1118,6 +1117,7 @@ function lowerStatement(
           consequent: loopBlock,
           alternate: continuationBlock.id,
           loc: stmt.node.loc ?? GeneratedSource,
+          fallthrough: continuationBlock.id,
         },
         continuationBlock,
       );
@@ -1203,6 +1203,7 @@ function lowerStatement(
           test,
           consequent: loopBlock,
           alternate: continuationBlock.id,
+          fallthrough: continuationBlock.id,
           loc: stmt.node.loc ?? GeneratedSource,
         },
         continuationBlock,
@@ -1800,6 +1801,7 @@ function lowerExpression(
           test: {...testPlace},
           consequent: consequentBlock,
           alternate: alternateBlock,
+          fallthrough: continuationBlock.id,
           id: makeInstructionId(0),
           loc: exprLoc,
         },
@@ -1878,6 +1880,7 @@ function lowerExpression(
           test: {...leftPlace},
           consequent,
           alternate,
+          fallthrough: continuationBlock.id,
           id: makeInstructionId(0),
           loc: exprLoc,
         },
@@ -2611,6 +2614,7 @@ function lowerOptionalMemberExpression(
       test: {...object},
       consequent: consequent.id,
       alternate,
+      fallthrough: continuationBlock.id,
       id: makeInstructionId(0),
       loc,
     };
@@ -2750,6 +2754,7 @@ function lowerOptionalCallExpression(
       test: {...testPlace},
       consequent: consequent.id,
       alternate,
+      fallthrough: continuationBlock.id,
       id: makeInstructionId(0),
       loc,
     };
@@ -4025,6 +4030,7 @@ function lowerAssignment(
           test: {...test},
           consequent,
           alternate,
+          fallthrough: continuationBlock.id,
           id: makeInstructionId(0),
           loc,
         },

compiler/packages/babel-plugin-react-compiler/src/HIR/HIR.ts

@@ -491,7 +491,7 @@ export type BranchTerminal = {
   alternate: BlockId;
   id: InstructionId;
   loc: SourceLocation;
-  fallthrough?: never;
+  fallthrough: BlockId;
 };
 
 export type SwitchTerminal = {

compiler/packages/babel-plugin-react-compiler/src/HIR/visitors.ts

@@ -660,11 +660,13 @@ export function mapTerminalSuccessors(
     case 'branch': {
       const consequent = fn(terminal.consequent);
       const alternate = fn(terminal.alternate);
+      const fallthrough = fn(terminal.fallthrough);
       return {
         kind: 'branch',
         test: terminal.test,
         consequent,
         alternate,
+        fallthrough,
         id: makeInstructionId(0),
         loc: terminal.loc,
       };
@@ -883,7 +885,6 @@ export function terminalHasFallthrough<
 >(terminal: T): terminal is U {
   switch (terminal.kind) {
     case 'maybe-throw':
-    case 'branch':
     case 'goto':
     case 'return':
     case 'throw':
@@ -892,6 +893,7 @@ export function terminalHasFallthrough<
       const _: undefined = terminal.fallthrough;
       return false;
     }
+    case 'branch':
     case 'try':
     case 'do-while':
     case 'for-of':

compiler/packages/babel-plugin-react-compiler/src/Inference/DropManualMemoization.ts

@@ -476,3 +476,12 @@
     }
   }
 }
+
+function findOptionalPlaces(fn: HIRFunction): Set<IdentifierId> {
+  const optionals = new Set<IdentifierId>();
+  for (const [, block] of fn.body.blocks) {
+    if (block.terminal.kind === 'optional') {
+    }
+  }
+  return optionals;
+}

compiler/packages/babel-plugin-react-compiler/src/ReactiveScopes/AlignReactiveScopesToBlockScopesHIR.ts

@@ -140,7 +140,7 @@ export function alignReactiveScopesToBlockScopesHIR(fn: HIRFunction): void {
     }
 
     const fallthrough = terminalFallthrough(terminal);
-    if (fallthrough !== null) {
+    if (fallthrough !== null && terminal.kind !== 'branch') {
       /*
        * Any currently active scopes that overlaps the block-fallthrough range
        * need their range extended to at least the first instruction of the