Verify valid users of Function

Summary:
`Function` only has a few instructions which are allowed to use it
directly, so put that in the IRVerifier.

Reviewed By: neildhar

Differential Revision: D69187679

fbshipit-source-id: a2b407fb94cd8f1da913a0544c3dfec788065064

lib/IR/IRVerifier.cpp

@@ -248,6 +248,15 @@ bool Verifier::visitFunction(const Function &F) {
         "Only GetNewTargetInst may use the newTargetParam");
   }
 
+  for (Instruction *user : F.getUsers()) {
+    AssertIWithMsg(
+        (*user),
+        llvh::isa<BaseCallInst>(user) ||
+            llvh::isa<BaseCreateLexicalChildInst>(user) ||
+            llvh::isa<GetClosureScopeInst>(user),
+        "Function can only be an operand to certain instructions");
+  }
+
   FunctionState newFunctionState(this, F);
 
   // Verify all basic blocks are valid