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

[Kai Luo via llvm-commits]

bzEq wrote:

> @bzEq could you please post it and add it to this patch to show the benefit?

Added [llvm/test/CodeGen/PowerPC/expand-isel-to-branch.ll](https://github.com/llvm/llvm-project/pull/72845/files#diff-8550e7b1ad6ad4554dca6b96b772f29cd2f6d7d3da509dbe79554e1474fef8fa).

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