[AMDGPU] Select no-return atomic ops in BUFInstructions.td

This change adds the selection of no-return buffer_* instructions in
tblgen. The motivation for this is to get the no-return atomic isel
working without relying on post-isel hooks so that GlobalISel can start
selecting them (once GlobalISelEmitter allows no return atomic patterns
like how DAGISel does).

This change handles the selection of no-return mubuf_atomic_cmpswap in
tblgen without changing the extract_subreg generation for the return
variant. This handling was done by the post-isel hook.

Differential Revision: https://reviews.llvm.org/D120538