[llvm] [PPC] Lower unreachable IR instruction to a trap. (PR #101379)

[Chen Zheng via llvm-commits]

https://github.com/chenzheng1030 approved this pull request.

LGTM except one concern for the case change. It is OK for me with either option, change the case or add the option.

https://github.com/llvm/llvm-project/pull/101379