[ValueTracking] Support signed intrinsic clamp

This is the same special logic we apply for SPF signed clamps
when computing the number of sign bits, just for intrinsics.

This just uses the same logic as the select case, but there's
multiple directions this could be improved in: We could also use
the num sign bits from the clamped value, we could do this during
constant range calculation, and there's probably unsigned analogues
for the constant range case at least.