Implement lowering of va_arg in clang directly. (This is 32-bit X86 only for now).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@58681 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/CGExprScalar.cpp b/lib/CodeGen/CGExprScalar.cpp
index 487cfcc..a4c65c0 100644
--- a/lib/CodeGen/CGExprScalar.cpp
+++ b/lib/CodeGen/CGExprScalar.cpp
@@ -1149,8 +1149,14 @@
 Value *ScalarExprEmitter::VisitVAArgExpr(VAArgExpr *VE) {
   llvm::Value *ArgValue = EmitLValue(VE->getSubExpr()).getAddress();
 
-  llvm::Value *V = Builder.CreateVAArg(ArgValue, ConvertType(VE->getType()));  
-  return V;
+  llvm::Value *ArgPtr = CGF.EmitVAArg(ArgValue, VE->getType());
+
+  // If EmitVAArg fails, we fall back to the LLVM instruction.
+  if (!ArgPtr) 
+    return Builder.CreateVAArg(ArgValue, ConvertType(VE->getType()));
+
+  // FIXME: volatile?
+  return Builder.CreateLoad(ArgPtr);
 }
 
 Value *ScalarExprEmitter::VisitObjCEncodeExpr(const ObjCEncodeExpr *E) {