[InstSimplify] handle all icmp i1 X, C in one place; NFCI

We already handled all of the new tests identically, but several
of those went through a lot of unnecessary processing before
getting folded.

Another motivation for grouping these cases together is that
InstCombine needs a similar fold. Currently, it handles the
'not' cases inefficiently which can lead to bugs as described
in the post-commit comments of:
https://reviews.llvm.org/D32143 

llvm-svn: 303295