[HLSL] Adding a test change I forgot to add

This test just verifies that even at -O0 the buffer subscript operators
are inlined. The original change was
fb5baffc28.

clang/test/CodeGenHLSL/builtins/RWBuffer-subscript.hlsl

@@ -0,0 +1,16 @@
+// RUN: %clang_cc1 -triple dxil-pc-shadermodel6.0-compute -emit-llvm -o - -O0 %s | FileCheck %s
+
+RWBuffer<int> In;
+RWBuffer<int> Out;
+
+[numthreads(1,1,1)]
+void main(unsigned GI : SV_GroupIndex) {
+  Out[GI] = In[GI];
+}
+
+// Even at -O0 the subscript operators get inlined. The -O0 IR is a bit messy
+// and confusing to follow so the match here is pretty weak.
+
+// CHECK: define internal void @"?main@@YAXI@Z"
+// CHECK-NOT: call
+// CHECK: ret void