[MLIR][SPIRV] Added storage class constraint on global variable

Added a check for 'Function' storage class in `spv.globalVariable`
verifier since it only can be used with `spv.Variable`.

Reviewed By: antiagainst

Differential Revision: https://reviews.llvm.org/D84731

mlir/lib/Dialect/SPIRV/SPIRVOps.cpp

@@ -1955,8 +1955,13 @@ static LogicalResult verify(spirv::GlobalVariableOp varOp) {
   // SPIR-V spec: "Storage Class is the Storage Class of the memory holding the
   // object. It cannot be Generic. It must be the same as the Storage Class
   // operand of the Result Type."
-  if (varOp.storageClass() == spirv::StorageClass::Generic)
-    return varOp.emitOpError("storage class cannot be 'Generic'");
+  // Also, Function storage class is reserved by spv.Variable.
+  auto storageClass = varOp.storageClass();
+  if (storageClass == spirv::StorageClass::Generic ||
+      storageClass == spirv::StorageClass::Function) {
+    return varOp.emitOpError("storage class cannot be '")
+           << stringifyStorageClass(storageClass) << "'";
+  }
 
   if (auto init =
           varOp.getAttrOfType<FlatSymbolRefAttr>(kInitializerAttrName)) {

mlir/test/Dialect/SPIRV/structure-ops.mlir

@@ -347,6 +347,13 @@ spv.module Logical GLSL450 {
 
 // -----
 
+spv.module Logical GLSL450 {
+  // expected-error @+1 {{storage class cannot be 'Function'}}
+  spv.globalVariable @var0 : !spv.ptr<f32, Function>
+}
+
+// -----
+
 spv.module Logical GLSL450 {
   spv.func @foo() "None" {
     // expected-error @+1 {{op must appear in a module-like op's block}}