[mlir] Add subbyte emulation support for `memref.store`. (#73174)

This adds a conversion for narrow type emulation of memref.store ops.
The conversion replaces the memref.store with two memref.atomic_rmw ops.
Atomics are used to prevent race conditions on same-byte accesses, in
the event that two threads are storing into the same byte.

Fixes https://github.com/openxla/iree/issues/15370