[InstCombine] (X != C1 && X != C2) --> (X | (C1 ^ C2)) != C2

...when C1 differs from C2 by one bit and C1 <u C2:
http://rise4fun.com/Alive/Vuo

And move related folds to a helper function. This reduces code duplication and
will make it easier to remove the scalar-only restriction as a follow-up step.

llvm-svn: 300364