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

[Nemanja Ivanovic via llvm-commits]

https://github.com/nemanjai commented:

I don't remember the details, but I remember that we decided to implement this as a late pass and maximally produce selects when the pass was first implemented. Unfortunately, the information seems not to have been preserved.
Perhaps it was to allow us to add intelligence to the pass about which ISEL instructions to convert to branches and which to leave based on HW performance characteristics. 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, so perhaps it is time to retire this pass and do what you're suggesting here.

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).

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