[MLIR] FlatAffineConstraints::isIntegerEmpty: fix bug in computation of duals

The method that was previously used for computing dual variables was incorrect.
This was used in the integer emptiness check algorithm, where this bug could lead to much longer running times. (Due to the way it is used, this never results in an incorrect emptiness check result.)

This patch fixes the dual computation and adds some additional asserts that catch this bug, along with regression test cases that trigger the asserts when the incorrect dual computation is used.

Reviewed By: Groverkss

Differential Revision: https://reviews.llvm.org/D113803