For SPIR-V 1.6 HelperInvocation accesses need to be volatile to avoid
undefined values when shaders execute 'demote'. Previously this was
always decorated on the gl_HelperInvocation variable, but this is not
valid when the Vulkan memory model is in use.
When the memory model is enabled, stop decorating the variable
declaration and apply the memory semantic to access chain loads instead.
Fixes#3042
1f233fa5c9