[MLIR][Affine] Allow affine-expr on RHS in IntegerSet

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
the RHS:

```
affine-constraint ::= affine-expr `>=` `affine-expr`
                    | affine-expr `==` `affine-expr`
```

The internal storage and printing of IntegerSet is still in the original format.

Reviewed By: bondhugula

Differential Revision: https://reviews.llvm.org/D128915