[RISCV][NFC] Refactor RISC-V vector intrinsic utils.

This patch is preparation for D111617, use class/struct/enum rather than char/StringRef to present internal information as possible, that provide more compact way to store those info and also easier to serialize/deserialize.

And also that improve readability of the code, e.g. "v" vs TypeProfile::Vector.

Reviewed By: khchen

Differential Revision: https://reviews.llvm.org/D124730