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

[Philip Reames via llvm-commits]

preames wrote:

> 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.

A while ago, I'd explored the idea of splitting "broadcast from scalar" and "broadcast from vector lane" in the costing interface.  I don't remember exactly why I stopped, but that might be relevant here.

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