[llvm] [RISCV] Unify getDemanded between forward and backwards passes in RISCVInsertVSETVLI (PR #92860)

[Piyou Chen via llvm-commits]

https://github.com/BeMg approved this pull request.

LGTM. The regression found by https://github.com/llvm/llvm-project/pull/70549 on `fixed-vectors-mask-buildvec.ll` can be fixed by this patch.

https://github.com/llvm/llvm-project/pull/92860