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

[Alexander Kornienko via llvm-commits]

alexfh wrote:

The reproducer @joanahalili mentioned above: https://godbolt.org/z/Eq36rPKc3

And a partially reduced standalone version in case it's more convenient to work with: https://gcc.godbolt.org/z/vKWrzc7KP

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