[InstCombine] allow vector splat matching for bitwise logic fold

Similar to 54e969cf (and with cosmetic updates to hopefully
make that easier to read), this fold has been around since early
in LLVM history.

Intermediate folds have been added subsequently, so extra uses
are required to exercise this code.

The test example actually shows an unintended consequence with
extra uses - we end up with an extra instruction compared to what
we started with. But this at least makes scalar/vector consistent.

General proof:
https://alive2.llvm.org/ce/z/tmuBza