[InstCombine] Make (X|C1)^C2 -> X^(C1^C2) iff X&~C1 == 0 work for splat vectors

This also corrects the description to match what was actually implemented. The old comment said X^(C1|C2), but it implemented X^((C1|C2)&~(C1&C2)). I believe ((C1|C2)&~(C1&C2)) is equivalent to (C1^C2).

Differential Revision: https://reviews.llvm.org/D36505

llvm-svn: 310658