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

[Kai Luo via llvm-commits]

bzEq wrote:

> But of course, the CPU's for which ISEL is sometimes very slow are mostly becoming less of a concern - plus we never implemented such heuristics

`SelectOptimizePass`, which is target independent, was introduced last year and is able to play this role(decide to use branch or `isel`), however it's not enabled by default on PowerPC. I recently did some investigation of this pass and have observed improvement in some benchmarks in SPEC2017 when pgo+lto is enabled.

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