[llvm] [AMDGPU] Allow SCC-only implicit defs and mark SOP2 SALU as rematerializable (PR #187084)

[Shilei Tian via llvm-commits]

https://github.com/shiltian commented:

LGTM but please wait for @arsenm for the final stamp since he is fine with this hook.

https://github.com/llvm/llvm-project/pull/187084