[llvm] [RISCV][CodeGen][TTI]Use processShuffleMasks for cost estimations/actual per-register shuffles (PR #118103)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

@alexey-bataev If you can pull out the x86 / processShuffleMasks changes - I can complete some wip x86 changes I have to use it for SK_PermuteTwoSrc

https://github.com/llvm/llvm-project/pull/118103