[RISCV] Use correct LMUL!=1 types for __attribute__((riscv_rvv_vector_bits(N)))

We used to convert them to M1 types in arguments and return
value, which causes failures in CodeGen since it is not legal
to insert subvectors with LMUL>1 to M1 vectors.

Fixes 64266

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D156779