PolynomialToModArith: branch lowerings based on eval form vs coeff form

polynomial.mul currently lowers via `polynoimal-to-mod-arith` to a convolution-based polynomial multiplication routine, assuming coefficient form. Now that we have NTT outputs as polynomial types with a "form" attribute, we need to support lowering a polynomial.mul op in `eval` form to an elementwise product with no ring reduction.

Similarly, we should review all the polynomial-to-mod-arith op lowerings and see which need to branch based on eval/coeff form differences in the input.