[llvm] PPC: Custom lower ppcf128 is_fpclass if is_fpclass is custom (PR #105540)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Aug 29, 5:54 AM EDT**: @arsenm started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/105540).


https://github.com/llvm/llvm-project/pull/105540