[RISCV] Enable fixed-length vectorization of LoopVectorizer for RISC-V Vector

By implementing the method "unsigned RISCVTTIImpl::getRegisterBitWidth(bool Vector)",
fixed-length vectorization is enabled when possible. Without this method, the
"#pragma clang loop" directive is needed to enable vectorization(or the cost model
may inform LLVM that "Vectorization is possible but not beneficial").

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D97549