polynomial.constant: transform argument coefficients to NTT form if needed

In PolyMulToNTT, the polynomial.constant op can be changed to have its output in eval form if needed. In this case, we should eagerly apply a compile-time NTT to the static coefficients array attached to the op, so that the semantics of the `polynomial.constant` op can be straightforwardly redefined as: "the operand are coefficients if the result type is in coeff form, and evaluations if the result type is in eval form."

We may want to do this for some other ops, like `polynomial.monomial`, but since `polynomial.monomial` is manly used as an interim implementation detail of the lowering of _coefficient_ form `polynomial.mul`, it is unlikely to be load bearing here. But `polynomial.constant` would participate in more meaningful optimizations, such as constant folding.