[MLIR][Presburger] fix bug where Simplex::addZeroRow was not undoable

[arjunp]

Summary

Previously, an UndoLogEntry was added by addRow but not by addZeroRow. So
calling directly into addZeroRow, as LexSimplex::addCut does, was not an
undoable operation. In the current usage of addCut this could never
lead to an incorrect result, and addZeroRow is protected, so it is not
currently possible to add a regression test for this. This bug needs to be
fixed for the symbolic integer lexmin algorithm.

[Groverkss]

LGTM.

mlir/lib/Analysis/Presburger/Simplex.cpp

 
 unsigned SimplexBase::addZeroRow(bool makeRestricted) {
   ++nRow;
   // If the tableau is not big enough to accomodate the extra row, we extend it.
   if (nRow >= tableau.getNumRows())
     tableau.resizeVertically(nRow);
   rowUnknown.push_back(~con.size());
   con.emplace_back(Orientation::Row, makeRestricted, nRow - 1);
+  undoLog.push_back(UndoLogEntry::RemoveLastConstraint);
 
   // Zero out the new row.
   tableau.fillRow(nRow - 1, 0);
 
   tableau(nRow - 1, 0) = 1;
   return con.size() - 1;
 }
 
for (unsigned i = 0; i < var.size(); ++i) {
     tableau(nRow - 1, 0) = lcm;
     for (unsigned col = 1; col < nCol; ++col)
       tableau(nRow - 1, col) =
           nRowCoeff * tableau(nRow - 1, col) + idxRowCoeff * tableau(pos, col);
   }
 
   normalizeRow(nRow - 1);
   // Push to undo log along with the index of the new constraint.
-  undoLog.push_back(UndoLogEntry::RemoveLastConstraint);
   return con.size() - 1;
 }
 
 /// Normalize the row by removing factors that are common between the
 /// denominator and all the numerator coefficients.
 void SimplexBase::normalizeRow(unsigned row) {
   int64_t gcd = 0;
   for (unsigned col = 0; col < nCol; ++col) {