[KnownBits] Implement knownbits `lshr`/`ashr` with exact flag

The exact flag basically allows us to set an upper bound on shift
amount when we have a known 1 in `LHS`.

Typically we deduce exact using knownbits (on non-exact incoming
shifts), so this is particularly impactful, but may be useful in some
circumstances.

Closes #84254