Coeff-form polynomial multiplication is broken

[crockeea]

!Z14604445665681409_i64 = !mod_arith.int<14604445665681409 : i64>
!rns_L0 = !rns.rns<!Z14604445665681409_i64>
#ring_rns_L0_1_x4096 = #polynomial.ring<coefficientType = !rns_L0, polynomialModulus = <1 + x**4096>>
!poly = !polynomial.polynomial<ring = #ring_rns_L0_1_x4096, form=coeff>
module attributes {GSW.input_abi_signature_intgt_logN12_bits2 = ["GSW", "GSW"], GSW.output_abi_signature_intgt_logN12_bits2 = ["RLWE"]} {
  func.func @intgt_logN12_bits2(%arg0: !poly) -> !poly {
    %36 = polynomial.mul %arg0, %arg0 : !poly
    return %36 : !poly
  }
}

with --polynomial-to-mod-arith, this outputs

tmpinput.mlir:5:1: error: unsupported coefficient type: '!rns.rns<!mod_arith.int<14604445665681409 : i64>>'
module attributes {GSW.input_abi_signature_intgt_logN12_bits2 = ["GSW", "GSW"], GSW.output_abi_signature_intgt_logN12_bits2 = ["RLWE"]} {
^
tmpinput.mlir:5:1: note: see current operation: %9 = "polynomial.constant"() <{value = #polynomial<typed_int_polynomial <1 + x**4096> : !polynomial.polynomial<ring = <coefficientType = !rns.rns<!mod_arith.int<14604445665681409 : i64>>, polynomialModulus = <-1 + x**8191>>>>}> : () -> !polynomial.polynomial<ring = <coefficientType = !rns.rns<!mod_arith.int<14604445665681409 : i64>>, polynomialModulus = <-1 + x**8191>>>
tmpinput.mlir:5:1: error: failed to legalize operation 'polynomial.constant' that was explicitly marked illegal: %9 = "polynomial.constant"() <{value = #polynomial<typed_int_polynomial <1 + x**4096> : !polynomial.polynomial<ring = <coefficientType = !rns.rns<!mod_arith.int<14604445665681409 : i64>>, polynomialModulus = <-1 + x**8191>>>>}> : () -> !polynomial.polynomial<ring = <coefficientType = !rns.rns<!mod_arith.int<14604445665681409 : i64>>, polynomialModulus = <-1 + x**8191>>>
module attributes {GSW.input_abi_signature_intgt_logN12_bits2 = ["GSW", "GSW"], GSW.output_abi_signature_intgt_logN12_bits2 = ["RLWE"]} {
^
tmpinput.mlir:5:1: note: see current operation: %9 = "polynomial.constant"() <{value = #polynomial<typed_int_polynomial <1 + x**4096> : !polynomial.polynomial<ring = <coefficientType = !rns.rns<!mod_arith.int<14604445665681409 : i64>>, polynomialModulus = <-1 + x**8191>>>>}> : () -> !polynomial.polynomial<ring = <coefficientType = !rns.rns<!mod_arith.int<14604445665681409 : i64>>, polynomialModulus = <-1 + x**8191>>>

I think this should be simple to fix; we recently added support for RNS in this pass, and this code branch is likely untested after the introduction of PolyMulToNTT.