[RISCV] Avoid generate large LMUL vmv.s.x or fvmv.s.f

This is a follow up to patch discussion on D139656. As noted there, M2/M4/M8 versions of these instructions don't actually exist, and using them results in overly constrained register allocation.

In that review, we'd talked about moving towards a variant of the instructions which ignored LMUL. I decided to see what happened if we just stopped generating the high LMUL variants, and the results are surprisingly neutral. I only see one minor thing which looks like a real regression among all the churn. I think this is worth doing now to loosen register allocation constraints, and avoid digging our hole around these instructions deeper while thinking about the right model change.

Differential Revision: https://reviews.llvm.org/D140027

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