[MLIR][Presburger] addSymbolicCut: fix the integral symbols heuristic to match the docs

Previously this checked if the entire symbolic numerator was divisible by the
denominator, which is never the case when this function is called. Fixed this to
check only the non-const coefficients in the numerator, which was what was
intended and documented.

Reviewed By: Groverkss

Differential Revision: https://reviews.llvm.org/D123592