Document SPIR-V work-group barrier requirements (OpControlBarrier)

[bashbaug]

See discussion: #1375 (comment)

We need to add a requirement in the SPIR-V environment spec to effectively say that an OpControlBarrier with work-group scope only has defined behavior when the barrier is encountered by all work-items in the work-group and within uniform control flow. This used to be documented in the SPIR-V specification, but it was removed in SPIR-V 1.6 revision 3 because it was too strict for some types of sub-group barriers in Vulkan.