[RISCV][MC] Warn if SEW/LMUL may not be compatible

According to RVV spec:
> In general, the requirement is to support LMUL ≥ SEWMIN/ELEN,
> where SEWMIN is the narrowest supported SEW value and ELEN is
> the widest supported SEW value.
>
> For a given supported fractional LMUL setting, implementations
> must support SEW settings between SEWMIN and LMUL * ELEN, inclusive.

We print a warning if these requirements are not met.

Reviewers: kito-cheng, asb, frasercrmck, jrtc27, michaelmaitland, lukel97

Reviewed By: lukel97

Pull Request: https://github.com/llvm/llvm-project/pull/94313