[SPIR-V] Fix crash in GetAttributeAtVertex impl.

tools/clang/lib/SPIRV/PervertexInputVisitor.cpp

@@ -56,6 +56,11 @@ int PervertexInputVisitor::appendIndexZeroAt(
 ///< treated as nointerpolated too.
 bool PervertexInputVisitor::expandNointerpVarAndParam(
     SpirvInstruction *spvInst) {
+  // If there is no AST type, it means the type was already lowered from some
+  // expression/construct. If it required expansion, it should be done already.
+  if (!spvInst->hasAstResultType())
+    return spvInst->isNoninterpolated();
+
   QualType type = spvInst->getAstResultType();
   bool isExpanded = false;
   auto typePtr = type.getTypePtr();

tools/clang/test/CodeGenSPIRV/intrinsics.get-attribute-at-vertex.cbuf.var.hlsl

@@ -0,0 +1,19 @@
+// RUN: dxc -T ps_6_2 -E main -spirv %s | FileCheck %s
+
+// CHECK:                                       OpDecorate %in_var_A PerVertexKHR
+// CHECK-DAG:                 %type_constants = OpTypeStruct %uint
+// CHECK-DAG:    %_ptr_Uniform_type_constants = OpTypePointer Uniform %type_constants
+// CHECK-DAG:            %_arr_v3float_uint_3 = OpTypeArray %v3float %uint_3
+// CHECK-DAG: %_ptr_Input__arr_v3float_uint_3 = OpTypePointer Input %_arr_v3float_uint_3
+cbuffer constants : register(b0)
+{
+  uint idx;
+}
+
+float4 main(nointerpolation float3 a : A) : SV_Target
+{
+// CHECK-DAG: %constants = OpVariable %_ptr_Uniform_type_constants Uniform
+// CHECK-DAG:  %in_var_A = OpVariable %_ptr_Input__arr_v3float_uint_3 Input
+  float value = GetAttributeAtVertex(a, 0)[idx];
+	return value.xxxx;
+}