Convert fastNTT to emit mod_arith ops directly

[j2kun]

In #1067 I did something slightly hacky while migrating polynomial-to-standard to use mod_arith ops: for the more complicated parts of the NTT lowerings, instead of lowering to mod_arith ops and types, I inserted conversion ops into and out of mod_arith at the start and end of ntt/intt. This allowed me to complete the polynomial type migration more quickly, but leaves that pass in a strange state where some conversions lower to mod_arith and others convert to arith.

This issue is to finish converting fastNTT to mod_arith.

[github-actions]

This issue has 3 outstanding TODOs:

• lib/Dialect/Polynomial/Conversions/PolynomialToModArith/PolynomialToModArith.cpp:1254: convert fastNTT to emit mod_arith ops.
• lib/Dialect/Polynomial/Conversions/PolynomialToModArith/PolynomialToModArith.cpp:1306: convert fastNTT to emit mod_arith ops.
• lib/Dialect/Polynomial/IR/PolynomialOps.cpp:237: use coefficientTypeMatchesScalarType for verifier to ensure the

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