[TTI] Add a Mask to getShuffleCost

This adds an Mask ArrayRef to getShuffleCost, so that if an exact mask
can be provided a more accurate cost can be provided by the backend.
For example VREV costs could be returned by the ARM backend. This should
be an NFC until then, laying the groundwork for that to be added.

Differential Revision: https://reviews.llvm.org/D98206