Reapply "[LV] Enable scalable FindLast on RISCV. (#184931)" (#190938)
Orginal PR: https://github.com/llvm/llvm-project/pull/184931
Original PR message:
This patch enables FindLast reduction vectorization with scalable
vectors
on RISCV.
Fixed the previous crash by
https://github.com/llvm/llvm-project/pull/191166 and
https://github.com/llvm/llvm-project/pull/191517