[InstCombine] add helper to foldXorOfICmps(); NFCI

Also, fix the old-style capitalization of the related functions
and move them to the 'private' section of the class since they
are just helpers of the visit* functions.

As shown in the post-commit comments for D32143, we are missing
folds for xor-of-icmps. 

llvm-svn: 303381