Constant folding for loop bounds.

- Fold the lower/upper bound of a loop to a constant whenever the result of the
  application of the bound's affine map on the operand list yields a constant.

- Update/complete 'for' stmt's API to set lower/upper bounds with operands.
  Resolve TODOs for ForStmt::set{Lower,Upper}Bound.

- Moved AffineExprConstantFolder into AffineMap.cpp and added
  AffineMap::constantFold to be used by both AffineApplyOp and
  ForStmt::constantFoldBound.

PiperOrigin-RevId: 215997346