[MLIR][Presburger] IntegerPolyhedron: add support for symbolic integer lexmin
Add support for computing the symbolic integer lexmin of a polyhedron.
This finds, for every assignment to the symbols, the lexicographically
minimum value attained by the dimensions. For example, the symbolic lexmin
of the set
`(x, y)[a, b, c] : (a <= x, b <= x, x <= c)`
can be written as
```
x = a if b <= a, a <= c
x = b if a < b, b <= c
```
This also finds the set of assignments to the symbols that make the lexmin unbounded.
This was previously landed in
da92f92621e28a56fe8ad79d82eb60e436bf1d39 and
reverted in
b238c252e8b1bbebc7ed79c08e06c23514d0dfb4 due to a build failure
in the code. Re-landing now with a fixed build.
Reviewed By: Groverkss
Differential Revision: https://reviews.llvm.org/D122985