JIT: ensure we keep throw blocks cold when combining blocks (dotnet#8…

…3842)

Noticed in a bunch of diffs that there were throw blocks intermixed
with regular blocks. Turns out when compacting a block followed by
a throw, we'd lose track of the fact that the combination should be cold.

src/coreclr/jit/fgopt.cpp

@@ -2224,35 +2224,43 @@ void Compiler::fgCompactBlocks(BasicBlock* block, BasicBlock* bNext)
         }
     }
 
-    // If either block or bNext has a profile weight
+    // If bNext is BBJ_THROW, block will become run rarely.
+    //
+    // Otherwise, if either block or bNext has a profile weight
     // or if both block and bNext have non-zero weights
     // then we will use the max weight for the block.
     //
-    const bool hasProfileWeight = block->hasProfileWeight() || bNext->hasProfileWeight();
-    const bool hasNonZeroWeight = (block->bbWeight > BB_ZERO_WEIGHT) || (bNext->bbWeight > BB_ZERO_WEIGHT);
-
-    if (hasProfileWeight || hasNonZeroWeight)
+    if (bNext->bbJumpKind == BBJ_THROW)
+    {
+        block->bbSetRunRarely();
+    }
+    else
     {
-        weight_t const newWeight = max(block->bbWeight, bNext->bbWeight);
+        const bool hasProfileWeight = block->hasProfileWeight() || bNext->hasProfileWeight();
+        const bool hasNonZeroWeight = (block->bbWeight > BB_ZERO_WEIGHT) || (bNext->bbWeight > BB_ZERO_WEIGHT);
 
-        if (hasProfileWeight)
+        if (hasProfileWeight || hasNonZeroWeight)
         {
-            block->setBBProfileWeight(newWeight);
+            weight_t const newWeight = max(block->bbWeight, bNext->bbWeight);
+
+            if (hasProfileWeight)
+            {
+                block->setBBProfileWeight(newWeight);
+            }
+            else
+            {
+                assert(newWeight != BB_ZERO_WEIGHT);
+                block->bbWeight = newWeight;
+                block->bbFlags &= ~BBF_RUN_RARELY;
+            }
         }
+        // otherwise if either block has a zero weight we select the zero weight
         else
         {
-            assert(newWeight != BB_ZERO_WEIGHT);
-            block->bbWeight = newWeight;
-            block->bbFlags &= ~BBF_RUN_RARELY;
+            noway_assert((block->bbWeight == BB_ZERO_WEIGHT) || (bNext->bbWeight == BB_ZERO_WEIGHT));
+            block->bbSetRunRarely();
         }
     }
-    // otherwise if either block has a zero weight we select the zero weight
-    else
-    {
-        noway_assert((block->bbWeight == BB_ZERO_WEIGHT) || (bNext->bbWeight == BB_ZERO_WEIGHT));
-        block->bbWeight = BB_ZERO_WEIGHT;
-        block->bbFlags |= BBF_RUN_RARELY; // Set the RarelyRun flag
-    }
 
     /* set the right links */