[clang] [RISCV] Reduce the size of the index used for RVV intrinsics. NFC (PR #74906)

[Philip Reames via cfe-commits]

https://github.com/preames approved this pull request.

LGTM - though maybe use uint32_t?

Looking at this code, the whole Intrinsics map vs OverloadIntrinsic map structure loops to have heavy redundancy and could be greatly simplified.  Maybe a follow up?

https://github.com/llvm/llvm-project/pull/74906