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

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@310658 91177308-0d34-0410-b5e6-96231b3b80d8