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

[Nemanja Ivanovic via llvm-commits]

nemanjai wrote:

I asked a similar question on the previous patch that turned off insertion of `isel` in Early If Conversion...
What problem is this solving? Why is this advantageous over the previous solution to always generate `isel` and then eliminate it if it needs to be eliminated?

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