[NFC][TTI] Add/extract `getReplicationShuffleCost()` method, deduplicate it's implementations

Hiding it in `getInterleavedMemoryOpCost()` is problematic for a number of reasons,
including testability and reuse, let's do better.

In a followup `getUserCost()` will be taught to use to to estimate the mask costs,
which will allow for better cost model tests for it.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D113313