Currently there is only one-way transform from KnownBits to ConstantRange.
Having an opposite transform could be good for consistency/testing coverage.
This could be also be of limited use for exact deduction on right-shifts,
although that will be limited to constant LHS, and won't fit within
existing ConstantRange::makeGuaranteedNoWrapRegion() interface
(it expects RHS and provides possible LHS'es, while there
we'd provide LHS and as about possible RHS's)
Also, this could be used to improve bitwise binary ops ConstantRange modelling (binaryAnd(), binaryOr(), binaryXor())