[MLIR] Simplex: fix a bug when rolling back a Simplex with no solutions

Previously, when adding a constraint to a Simplex that is already marked
as having no solutions (marked empty), the Simplex would be marked empty again,
and a second UnmarkEmpty entry would be pushed to the undo log. When rolling
back, Simplex should be unmarked empty only after rolling back past the
creation of the first constraint that made it empty.

Reviewed By: Groverkss

Differential Revision: https://reviews.llvm.org/D114613