[X86] Make `(shift X (xor/sub N-1, Y))` -> `(shift X, (not Y))` check for one use.

`(xor/sub N-1, Y)` -> `(not Y)` is minorly preferable (especially for
`(sub N-1, Y)` where it saves an instruction), but isn't worth
potentially creating an extra instruction for.

So, only do the transformation if `(xor/sub N-1, Y)` has one use.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D144985