[clang][bytecode] Load value of non-lvalue ArraySubscriptExpr (#160024)

As happens in C.

Fixes #158482

Author: tbaederr

clang/lib/AST/ByteCode/Compiler.cpp

@@ -1787,7 +1787,12 @@ bool Compiler<Emitter>::VisitArraySubscriptExpr(const ArraySubscriptExpr *E) {
     return false;
   if (DiscardResult)
     return this->emitPopPtr(E);
-  return true;
+
+  if (E->isGLValue())
+    return true;
+
+  OptPrimType T = classifyPrim(E);
+  return this->emitLoadPop(*T, E);
 }
 
 template <class Emitter>

clang/test/AST/ByteCode/c.c

@@ -368,3 +368,7 @@ void discardedCmp(void)
 {
     (*_b) = ((&a == &a) , a); // all-warning {{left operand of comma operator has no effect}}
 }
+
+/// ArraySubscriptExpr that's not an lvalue
+typedef unsigned char U __attribute__((vector_size(1)));
+void nonLValueASE(U f) { f[0] = f[((U)(U){0})[0]]; }