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

This change adds the selection for the no-return global_* and flat_*
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).

Differential Revision: https://reviews.llvm.org/D119227