[llvm] [PowerPC]optimize the epilogue for restore non volatile cr fields (PR #202339)

[Hubert Tong via llvm-commits]

hubert-reinterpretcast wrote:

> so if there only restore one CR field, using mtocrf has better performance than mtcrf. if there are two and more CR fields are restored, the mtcrf is used.

The quoted text suggests that multiple `mtocrf`s may still be better than a single `mtcrf` that modifies a _subset_ of the fields; however, I suggested `mtcr` (which modifies _all_ of the fields).

https://github.com/llvm/llvm-project/pull/202339