[llvm] [SLP]Add cost estimation for gather node reshuffling (PR #115201)

[Simon Pilgrim via llvm-commits]

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

LGTM - my only minor concern is that a lot of targets' cost tables assume SK_Broadcast Index == 0, but we can address that in TTI fixes as necessary.

https://github.com/llvm/llvm-project/pull/115201