[Clang][CodeGen] Address review comments. NFC.

Author: dtcxzyw

clang/lib/CodeGen/CGExprConstant.cpp

@@ -1883,10 +1883,10 @@ llvm::Constant *ConstantEmitter::tryEmitPrivateForVarInit(const VarDecl &D) {
 
   // Try to emit the initializer.  Note that this can allow some things that
   // are not allowed by tryEmitPrivateForMemory alone.
-  if (APValue *value = D.evaluateValue()) {
-    if (!value->allowConstexprUnknown())
-      return tryEmitPrivateForMemory(*value, destType);
-  }
+  // Bail out on constexpr-unknown values since they are invalid in CodeGen.
+  if (APValue *value = D.evaluateValue();
+      value && !value->allowConstexprUnknown())
+    return tryEmitPrivateForMemory(*value, destType);
 
   return nullptr;
 }