[knownbits] Preserve known bits for small shift recurrences

The motivation for this is that I'm looking at an example that uses shifts as induction variables. There's lots of other omissions, but one of the first I noticed is that we can't compute tight known bits. (This indirectly causes SCEV's range analysis to produce very poor results as well.)

Differential Revision: https://reviews.llvm.org/D96440