[llvm] [PowerPC] improve performance on the isNan and !isNan function in case of -ffp-model=strict" (PR #215587)

[zhijian lin via llvm-commits]

diggerlin wrote:

the PR fix the bugs introduced by  the [Reland [PowerPC] improve performance on the isNan and !isNan function in case of -ffp-model=strict](https://github.com/llvm/llvm-project/pull/212565#top) , the bugs are description in PR https://github.com/llvm/llvm-project/pull/213298 

https://github.com/llvm/llvm-project/pull/215587