[RISCV] Use v8-v23 as argument registers to conform to the proposal.

The maximum LMUL is 8. We need 16 vector registers for two LMUL-8
arguments. The modification follows the proposal of psABI in
https://github.com/riscv/riscv-elf-psabi-doc/pull/171

Differential Revision: https://reviews.llvm.org/D95134