Convert expr - c * (expr floordiv c) to expr mod c in AffineExpr

- Detect 'mod' to replace the combination of floordiv, mul, and subtract when
  possible at construction time; when 'c' is a power of two, this reduces the number of
  operations; also more compact and readable. Update simplifyAdd for this.

  On a side note:
  - with the affine expr flattening we have, a mod expression like d0 mod c
    would be flattened into d0 - c * q,  c * q <= d0 <= c*q + c - 1, with 'q'
    being added as the local variable (q = d0 floordiv c); as a result, a mod
    was turned into a floordiv whenever the expression was reconstructed back,
    i.e., as  d0 - c * (d0 floordiv c); as a result of this change, we recover
    the mod back.

- rename SimplifyAffineExpr -> SimplifyAffineStructures (pass had been renamed but
  the file hadn't been).

PiperOrigin-RevId: 228258120