[MLIR][Presburger] Simplex: remove redundant zeroing out of row

This fillRow(..., 0) is redundant because when the size of the tableau is
consistent, the resize always creates a new row, which is zero-initialized.

Also added asserts throughout to ensure the dimensions of the tableau remain
consistent.

Reviewed By: Groverkss

Differential Revision: https://reviews.llvm.org/D126709