[ConstraintElim] Use SCEV to check for multiples (#76925)

When adding constraints for induction variables, if the step is not one,
we need to make sure that (end-start) is a multiple of step, otherwise
we might step over the end value.

Currently this only supports one specific pattern for pointers, where
the end is a gep of the start with an appropriate offset.

Generalize this by using SCEV to check for multiples, which also makes
this work for integer IVs.