Support for affine integer sets

- introduce affine integer sets into the IR
- parse and print affine integer sets (both inline or outlined) similar to
  affine maps
- use integer set for IfStmt's conditional, and implement parsing of IfStmt's
  conditional

- fixed an affine expr paren omission bug while one this.

TODO: parse/represent/print MLValue operands to affine integer set references.
PiperOrigin-RevId: 207779408