[spirv] Legalization: indexing StructuredBuffer containing array (#982)

We need to call loadIfAliasVar() to set the correct storage class
and layout rule for the pointer type.

tools/clang/lib/SPIRV/SPIRVEmitter.cpp

@@ -1569,7 +1569,7 @@ SPIRVEmitter::doArraySubscriptExpr(const ArraySubscriptExpr *expr) {
   TypeTranslator::LiteralTypeHint hint(typeTranslator, astContext.IntTy);
 
   llvm::SmallVector<uint32_t, 4> indices;
-  auto info = doExpr(collectArrayStructIndices(expr, &indices));
+  auto info = loadIfAliasVarRef(collectArrayStructIndices(expr, &indices));
 
   if (!indices.empty()) {
     (void)turnIntoElementPtr(info, expr->getType(), indices);

tools/clang/test/CodeGenSPIRV/spirv.legal.sbuffer.usage.hlsl

@@ -2,6 +2,7 @@
 
 struct S {
     float4 f;
+    float  v[128];
 };
 
 // Signature for returnRWSBuffer(). Both the function parameter and return gain
@@ -61,6 +62,10 @@ float4 main(in float4 pos : SV_Position) : SV_Target
 // CHECK-NEXT: [[ptr2:%\d+]] = OpAccessChain %_ptr_Uniform_v4float [[ptr1]] %int_0 %uint_2 %int_0
 // CHECK-NEXT:      {{%\d+}} = OpLoad %v4float [[ptr2]]
         localRWSBuffer[2].f +
+// CHECK:      [[ptr1:%\d+]] = OpLoad %_ptr_Uniform_type_RWStructuredBuffer_S %localRWSBuffer
+// CHECK-NEXT: [[ptr2:%\d+]] = OpAccessChain %_ptr_Uniform_float [[ptr1]] %int_0 %uint_33 %int_1 %int_44
+// CHECK-NEXT:      {{%\d+}} = OpLoad %float [[ptr2]]
+        localRWSBuffer[33].v[44] +
 // CHECK:      [[ptr1:%\d+]] = OpLoad %_ptr_Uniform_type_RWStructuredBuffer_v4float %localv4f32RWSBuffer
 // CHECK-NEXT: [[ptr2:%\d+]] = OpAccessChain %_ptr_Uniform_v4float [[ptr1]] %int_0 %uint_3
 // CHECK-NEXT:      {{%\d+}} = OpLoad %v4float [[ptr2]]