[RISCV] Cost model for general case of dual vector permute

The cost model was not accounting for the fact that we can generate a dual vrgather + an index expression sequence instead of scalarizing.

A couple cases to call out:

1) I did not model the difference between vrgather and vrgatherei16. The result is the constant pool cost can be slightly understated on RV32. I don't think we care, but if someone disagrees, this would be easy to add.
2) Our current codegen for i8 vectors longer than 256 (which is the limit of what this costs) has some room for improvement.
3) As indicated by the *regression* in reported cost for <2 x iN> vectors, our current vector lowering is missing support for a sub-case where scalarize-and-insert is actually faster than the generic fallback path.

Differential Revision: https://reviews.llvm.org/D147063

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