[clang] [RISCV] Refactor RVV builtin code generation for reduce compilation time [NFC] (PR #154906)

[Kito Cheng via cfe-commits]

kito-cheng wrote:

Changes:
- Address Craig's comment
  - Drop else after return
  - Adjust size for SmallVector
  - Use SmallVectorImpl at argument list
  - Use array to CGM.getIntrinsic's argument directly rather than create a temporary SmallVector

https://github.com/llvm/llvm-project/pull/154906