[llvm] [PowerPC] Do not generate `isel` instruction if target doesn't have this instruction (PR #72845)

[Chen Zheng via llvm-commits]

https://github.com/chenzheng1030 approved this pull request.

> I don't see regression in SPEC2017 when -mno-isel and -bplugin_opt:-mattr=-isel are specified with PPCExpandISEL removed from the backend pipeline.
I'll investigate more with PPCExpandISEL removed and will post another patch to disable it if no further issue is found.

LGTM. Thanks. And the plan to remove `PPCExpandISEL` pass also looks good to me.

https://github.com/llvm/llvm-project/pull/72845