[InstCombine] remove duplicated fold ; NFCI

This pattern is matched in foldICmpBinOpEqualityWithConstant() and already works
with vectors too. I changed some comments over there to point out the current 
location. The tests for this transform are currently in 'sub.ll'.

Note that the remaining folds in this block all require a sub too, so they should
get grouped with the other icmp(sub) patterns.

llvm-svn: 281627