microsoft/compiler: Un-lower shared/scratch to derefs

microsoft/compiler: Un-lower shared/scratch to derefs

Derefs have index-based access semantics, which means we don't need
custom intrinsics to encode an index instead of a byte offset.

Remove the "masked" store intrinsics and just emit the pair of atomics
directly. This massively reduces duplication between scratch, shared,
and constant, while also moving more things into nir so more optimizations
can be done.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23173>