[RISCV] Avoid using x0,x0 vsetvli for vmv.x.s and vfmv.f.s unless we know the sew...

[RISCV] Avoid using x0,x0 vsetvli for vmv.x.s and vfmv.f.s unless we know the sew/lmul ratio is constant.

Since we're changing VTYPE, we may change VLMAX which could
invalidate the previous VL. If we can't tell if it is safe we
should use an AVL of 1 instead of keeping the old VL.

This is a quick fix. We may want to thread VL to the pseudo
instruction instead of making up a value. That will require ISD
opcode changes and changes to the C intrinsic interface.

This fixes the issue raised in D106286.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D106403