[RISCV][NFC] Change interface of RVVIntrinsic::getSuffixStr

This NFC patch is splited from D111617.

Using llvm::ArrayRef rather than llvm::SmallVector, ArrayRef is more generic
interface that could accept both llvm::ArrayRef and llvm::SmallVector.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D125893