[ValueTracking] Determine assume KnownBits using ConstantRange

For non-equality icmps, we don't do any KnownBits-specific
reasoning, and just use the known bits as a constraint on the range.
We can generalize this for all predicates by round-tripping through
ConstantRange and using makeAllowedICmpRegion().

The minor improvement in zext-or-icmp is because we assume that
a value is ult [0,1], which means it must be zero.