Currently, the parser for IntegerSet, only allows constraints like:
affine-constraint ::= affine-expr `>=` `0` | affine-expr `==` `0`
This form is sometimes unreadable and painful to use when writing unittests
for Presburger library and tests in general.
This patch extends the parser to allow affine constraints with affine-expr on
affine-constraint ::= affine-expr `>=` `affine-expr` | affine-expr `==` `affine-expr`
The internal storage and printing of IntegerSet is still in the original format.