[llvm] [PowerPC] improve performance on the isNan and !isNan function in case of -ffp-model=strict (PR #204170)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clangd-ubuntu-tsan` running on `clangd-ubuntu-clang` while building `llvm` at step 2 "checkout".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/134/builds/44265

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 2 (checkout) failure: update (failure)
git version 2.17.1
Cloning into '.'...
fatal: unable to access 'https://github.com/llvm/llvm-project.git/': Could not resolve host: github.com
Cloning into '.'...
fatal: unable to access 'https://github.com/llvm/llvm-project.git/': Could not resolve host: github.com

```

</details>

https://github.com/llvm/llvm-project/pull/204170