[RISCV] Refactor RVV Policy by structure

RVV intrinsic function has several policy variants.

Include TU, TA, TAMU, TAMA, TUMU, TUMA, MU, MA, TUM, TAM

Currently, the clang side enumerates these policies, but it's hard to add a new policy.

This patch use structure to replace the origin policy enumeration, and enhance some policy transform logic.

This is a clean-up job that will not affect the RVV intrinsic functionality and make sure riscv_vector_builtin_cg.inc is the same as the original one.

Reviewed By: kito-cheng

Differential Revision: https://reviews.llvm.org/D139995