[ValueTracking] Improve ComputeNumSignBits for SRem.

The result will have the same sign as the dividend unless the
result is 0. The magnitude of the result will always be less
than or equal to the dividend. So the result will have at least
as many sign bits as the dividend.

Previously we would do this if the divisor was a positive constant,
but that isn't required.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D97170