[AMDGPU] Select buffer_atomic_cmpswap* in tblgen

This change replaces the manual selection of buffer_atomic_cmpswap*
instructions in SelectionDAG and GlobalISel with a tblgen based
selection in BUFInstructions.td. This allows us to select the return and
no-return variants in tblgen.

Differential Revision: https://reviews.llvm.org/D121770