Update SideEffect model for CreateClassInst

Summary:
It's only possible for `CreateClass` to have execute side effects
when it's a derived class constructor, because it fetches the
`.prototype` of the super class.

Reviewed By: tmikov

Differential Revision: D67404012

fbshipit-source-id: 9ca7af807f27ae127c342ece686d7cfcd42c199b

doc/IR.md

@@ -397,7 +397,7 @@ Description | Constructs a new class into the current scope from its constructor
 Example | %0 = CreateClassInst %scope, %function, %superClass, %homeObjectOutput
 Arguments | %function is the function that represents the code of the constructor. %scope is the surrounding environment. %superClass is the class to inherit from; in base classes this is an empty sentinel value. %homeObjectOutput is an out parameter which will contain the home object (.prototype) of the class.
 Semantics | The instruction creates a new class that may access the lexical scope of the current function, and an inherit from a given super class. (ES2023 15.7.14) This results in the creation of 2 objects: the class function object itself, and the "home" object. The home object is where methods are put. The home object can be found on the .prototype of the class, and the class can be found on the .constructor of the home object.
-Effects | May execute JS and write to stack memory.
+Effects | Writes to stack memory. May execute JS if it's a derived class.
 
 ### BinaryOperatorInst
 

include/hermes/IR/Instrs.h

@@ -1037,7 +1037,7 @@ class CreateClassInst : public BaseCreateCallableInst {
     return getOperand(HomeObjectOutIdx);
   }
 
-  Value *getSuperClass() {
+  Value *getSuperClass() const {
     return getOperand(SuperClassIdx);
   }
 
@@ -1046,10 +1046,14 @@ class CreateClassInst : public BaseCreateCallableInst {
   }
 
   SideEffect getSideEffectImpl() const {
-    // When creating a derived class, we look up the .prototype of the super
-    // class we are deriving from. This property look up can potentially trigger
-    // JS.
-    return SideEffect::createExecute().setWriteStack();
+    if (llvh::isa<EmptySentinel>(getSuperClass())) {
+      // When creating a derived class, we look up the .prototype of the super
+      // class we are deriving from. This property look up can potentially
+      // trigger JS.
+      return SideEffect::createExecute().setWriteStack();
+    } else {
+      return SideEffect{}.setReadHeap().setWriteStack();
+    }
   }
 
   static bool hasOutput() {