[llvm] [LV][VPlan] Add initial support for CSA vectorization (PR #121222)

[Michael Maitland via llvm-commits]

michaelmaitland wrote:

> Now that @Mel-Chen's recent changes landed, I'd hope that adding support for FindLast may be slightly easier, but I am not sure. Maybe @Mel-Chen has additional thoughts?

FindLast approach only works when things are monotonic. CSA works regardless of whether things are monotonic. Reductions kick in before CSA, so if we can vectorize using FindLast, we will vectorize it (and prefer it) compared to CSA.

https://github.com/llvm/llvm-project/pull/121222