[NFC][X86] combineX86ShuffleChain(): hoist Mask variable higher up

Having `NewMask` outside of an if and rebinding `BaseMask` `ArrayRef`
to it is confusing. Instead, just move the `Mask` vector higher up,
and change the code that earlier had no access to it but now does
to use `Mask` instead of `BaseMask`.

This has no other intentional changes.

This is a recommit of 35c0848b,
that was reverted to simplify reversion of an earlier change.