ValueTracking: Drop rounding mode check for constrained_sqrt in CannotBeNegativeZero

The only value that can produce -0 is exactly -0, no rounding is involved. If the
denormal mode has flushed denormal inputs, a negative value could produce -0.
The constrained intrinsics do not track the denormal mode, and this is just
generally broken in the current set of FP predicates. The move to computeKnownFPClass
will address some of these issues.