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

[Graham Hunter via llvm-commits]

huntergr-arm wrote:

As I mentioned in the recent community call, I have a version of this using a RecurrenceDescriptor instead of a new class: https://github.com/llvm/llvm-project/compare/main...huntergr-arm:llvm-project:csa-using-recurrence

It took a bit longer than expected to post this, as the state I'd left it in hadn't just changed the detection code but I had also started working on the transformation part. I've now completed a prototype of that. This would benefit from the work to unify the AnyOf recurrence -- having separate recurrence kinds for signed, unsigned and FP CSA instances (or other future recurrence types) is less than ideal.

I've added one of Michael's tests to the AArch64 directory since I'm more familiar with it. At least some of the existing tests in the RiscV directory still vectorize, but I haven't reimplemented everything.

What do you think of the approach?

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