[llvm] [WIP][TTI] Refactor improveShuffleKindFromMask to match the pattern order of getInstructionCost (PR #151984)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

@davemgreen You mentioned that you wanted to make it easier to handle shuffle narrow/widening costs at the target level - do you have specific examples of this?

https://github.com/llvm/llvm-project/pull/151984