[llvm] Reapply "[LV] Enable scalable FindLast on RISCV. (#184931)" (PR #190938)

[Luke Lau via llvm-commits]

https://github.com/lukel97 approved this pull request.

LGTM. I checked locally that when we get to `VPlanTransforms::addExplicitVectorLength` that all ComputeReductionResults with a select operand have their select operand in the loop body, not in the middle block. I.e. it should be compatible with EVL tail folding. I've added a PR to assert this in https://github.com/llvm/llvm-project/pull/191767

https://github.com/llvm/llvm-project/pull/190938