[MLIR][Presburger] subtract: fix support for divs defined by equalities

Also added test cases to test this. Both IntegerRelation::addLocalFloorDiv and the fixed implementation of subtraction need to compute division inequalities from dividend and divisor, so this also adds helper util functions to avoid duplicating this logic.

Reviewed By: Groverkss

Differential Revision: https://reviews.llvm.org/D128736