[MLIR][Presburger] Fix Gaussian elimination (#164437)

In the Presburger library, there are two minor bugs of Gaussian
elimination.

In Barvinok.cpp, the `if (equations(i, i) != 0) continue;` is intended
to skip only the row-swapping, but it in fact skipped the whole loop
body altogether, including the elimination parts.

In IntegerRelation.cpp, the Gaussian elimination forgets to advance
`firstVar` (the number of finished columns) when it finishes a column.
Moreover, when it checks the pivot row of each column, it didn't ignore
the rows considered.

As an example, suppose the constraints are
```
1 0 0 1 2 = 0
0 1 0 0 3 = 0
0 0 0 1 4 = 0
...
```
For the 4th column, it will think the pivot is the first row `1 0 0 1 2
= 0`, rather than the correct 3rd row `0 0 0 1 4 = 0`.

(This bug is left undiscovered, because if we don't advance `firstVar`
then this Gaussian elimination process will simply do nothing. Moreover,
it is called only in `simplify()`, and the existing test cases doesn't
care whether a set has been simplified.)