[AMDGPU] Insert spill codes for the SGPRs used for EXEC copy (#79428)

The SGPR registers used for preserving EXEC mask while lowering the
whole-wave register spills and copies should be preserved at the prolog
and epilog if they are in the CSR range. It isn't happening when there
is only wwm-copy lowered and there are no wwm-spills. This patch
addresses that problem.