[MLIR][Presburger] LexSimplex::addEquality: add equalities as fixed columns

In LexSimplex, instead of adding equalities as a pair of inequalities,
add them as a single row, move them into the basis, and keep them there.

There will always be a valid basis involving all non-redundant equalities. Such
equalities will then be ignored in some other operations, such as when looking
for pivot columns. This speeds them up a little bit.

More importantly, this is an important precursor patch to adding support for
symbolic integer lexmin, as this heuristic can sometimes make a big difference there.

Reviewed By: Groverkss

Differential Revision: https://reviews.llvm.org/D122165