[llvm] [VectorCombine] Use getInstructionCost to cost Shuffle. (PR #122068)

[Simon Pilgrim via llvm-commits]

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

LGTM - some of the other 'OldCost' calculations could probably benefit from something similar.

https://github.com/llvm/llvm-project/pull/122068