[MLIR][sparse][GPU] fixing windows build break caused by D151014

[K-Wu]

fixing compile error

[K-Wu]

fixing compile error

[aartbik]

mlir/lib/Dialect/GPU/IR/GPUDialect.cpp

176

I think you can omit the return after the unreachable

mlir/lib/Dialect/GPU/IR/GPUDialect.cpp

case SparseHandleKind::Env:
     return "sparse.env_handle";
   case SparseHandleKind::DnVec:
     return "sparse.dnvec_handle";
   case SparseHandleKind::DnMat:
     return "sparse.dnmat_handle";
   case SparseHandleKind::SpMat:
     return "sparse.spmat_handle";
   }
+  UNREACHABLE("unknown sparse handle kind");
+  return "";
 }
 
 Type GPUDialect::parseType(DialectAsmParser &parser) const {
   // Parse the main keyword for the type.
   StringRef keyword;
   if (parser.parseKeyword(&keyword))
     return Type();
   MLIRContext *context = getContext();