[ValueTracking] improve analysis for "C << X" and "C >> X"

This is based on the example/comments in:
https://llvm.org/PR48984

I tried just lifting the restriction in computeKnownBitsFromShiftOperator()
as suggested in the bug report, but that doesn't catch all of the cases
shown here. I didn't step through to see exactly why that happened. But it
seems like a reasonable compromise to cheaply check the special-case of
shifting a constant.

There's a slight regression on a cmp transform as noted, but this is likely
the more important/common pattern, so we can fix that icmp pattern later if
needed.

Differential Revision: https://reviews.llvm.org/D95959