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

[Chen Zheng via llvm-commits]

chenzheng1030 wrote:

> It would be nice if you could highlight some changes in the test cases that are clear wins with this approach (I wasn't able to spot any with a cursory glance at the test case changes).

I remembered we had a motivated case when -mno-isel was specified. @bzEq could you please post it and add it to this patch to show the benefit?

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