AffineExpr pretty print - add missing handling to print expr * - 1 as -expr

- print multiplication by -1 as unary negate; expressions like s0 * -1, d0 * -1
  + d1 will now appear as -s0, -d0 + d1 resp.
- a minor cleanup while on printAffineExprInternal

PiperOrigin-RevId: 230222151