[InstCombine] enable optimization of casted vector xor instructions

This is part of the payoff for the refactoring in:
http://reviews.llvm.org/rL261649
http://reviews.llvm.org/rL261707

In addition to removing a pile of duplicated code, the xor case was
missing the optimization for vector types because it checked
"SrcTy->isIntegerTy()" rather than "SrcTy->isIntOrIntVectorTy()"
like 'and' and 'or' were already doing.

This solves part of:
https://llvm.org/bugs/show_bug.cgi?id=26702

llvm-svn: 261750