[InstCombine] Fold (xor (min/max X, Y), -1) -> (max/min ~X, ~Y) when X and Y are freely invertible.

This allows the xor to be removed completely.

This might help with recomitting r341674, but seems good regardless.

Coincidentally fixes PR38915.

Differential Revision: https://reviews.llvm.org/D51964

llvm-svn: 342163