[llvm] Reland [PowerPC] improve performance on the isNan and !isNan function in case of -ffp-model=strict (PR #212565)

[Daniel Chen via llvm-commits]

DanielCChen wrote:

@diggerlin 
This PR caused external AIX buildbot to fail. I can also reproduce it on my local AIX 7.2 box. 
Did your pre-merge build actually pass on some AIX OS? Just wondering if it is a OS specific failure.
I opened https://github.com/llvm/llvm-project/pull/213298 to try to fix it. Please review. Thanks!

https://github.com/llvm/llvm-project/pull/212565