[InstCombine] add fold for icmp with or mask of low bits (PR32542)

We already have these 'and' folds:

// X & -C == -C -> X >  u ~C
// X & -C != -C -> X <= u ~C
//   iff C is a power of 2

...but we were missing the 'or' siblings.

http://rise4fun.com/Alive/n6

This should improve:
https://bugs.llvm.org/show_bug.cgi?id=32524
...but there are 2 or more other pieces to fix still.

Differential Revision: https://reviews.llvm.org/D31712

llvm-svn: 299570