OpcodeDispatcher: Add DeriveOp helper

The "create op with wrong opcode, then change the opcode" pattern is REALLY
dangerous. This does not address that. But when we start doing NZCV trickery, it
will get /more/ dangerous, and so it's time to add a helper and make the
convenient thing the safe(r) thing. This helper correctly saves NZCV /before/
the instruction like the real builders would. It also provides a spot for future
safety asserts if someone is motivated.

Signed-off-by: Alyssa Rosenzweig <[email protected]>

FEXCore/Source/Interface/Core/OpcodeDispatcher.h

@@ -1329,6 +1329,16 @@ friend class FEXCore::IR::PassManager;
     }
   }
 
+  // Helper to derive Dest by a given builder-using Expression with the opcode
+  // replaced with NewOp. Useful for generic building code. Not safe in general.
+  // but does the right handling of ImplicitFlagClobber at least and must be
+  // used instead of raw Op mutation.
+#define DeriveOp(Dest, NewOp, Expr) \
+  if (ImplicitFlagClobber(NewOp))   \
+    SaveNZCV();                     \
+  auto Dest = (Expr);               \
+  Dest.first->Header.Op = (NewOp)
+
   // Named constant cache for the current block.
   // Different arrays for sizes 1,2,4,8,16,32.
   OrderedNode *CachedNamedVectorConstants[FEXCore::IR::NamedVectorConstant::NAMED_VECTOR_MAX][6]{};