[OPT] Add extension to allow list

The SPV_GOOGLE_user_type extension was not added to one of the allow
list. Adding it now.

Part of microsoft/DirectXShaderCompiler#7181.

Author: s-perron

source/opt/local_single_store_elim_pass.cpp

@@ -150,7 +150,8 @@ void LocalSingleStoreElimPass::InitExtensionAllowList() {
                                 "SPV_AMDX_shader_enqueue",
                                 "SPV_KHR_fragment_shading_rate",
                                 "SPV_KHR_ray_tracing",
-                                "SPV_KHR_quad_control"});
+                                "SPV_KHR_quad_control",
+                                "SPV_GOOGLE_user_type" });
 }
 bool LocalSingleStoreElimPass::ProcessVariable(Instruction* var_inst) {
   std::vector<Instruction*> users;