Support for SPIR-V OpMemoryBarrier

MSL supports `OpControlBarrier` via `threadgroup_barrier()`, but MSL does not support the equivalent of `OpMemoryBarrier`, as distinct from this.

This becomes meaningful because `OpMemoryBarrier` does not have the same requirement for uniform flow (consistent `if` and `for` statement paths across threads) that govern `OpControlBarrier` and `threadgroup_barrier()`, and there are [use-cases](https://github.com/KhronosGroup/MoltenVK/issues/973) that depend on this.

@kvark commented:
> I double checked the specs and can confirm that Metal only supports uniform control flow barriers. A new flag for this might be needed, something like `nonUniformMemoryBarriers`.

Start by reviewing the existing OpControlBarrier handling through MSL’s threadgroup_barrier() and the distinction from OpMemoryBarrier. Consider the linked MoltenVK issue and whether a nonUniformMemoryBarriers flag is needed. Done means the portability layer supports the described non-uniform memory-barrier use cases without treating them as uniform control-flow barriers.

Written by the indexing model from the issue text.