[AMDGPU] Add optional tied-op for wwm-register's epilog spill restore

The COPY inserted in the epilog block before return instruction as part
of ABI lowering, can get optimized during machine copy propagation if
the same register is used earlier in a wwm operation that demands the
prolog/epilog wwm-spill store/restore to preserve its inactive lanes.
With the spill restore in the epilog, the preceding COPY appears to be
dead during machine-cp. To avoid it, mark the same register as a tied-op
in the spill restore instruction to ensure a usage for the COPY.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D150381