[RISCV] Fold ops into vmv.v.v as vmerge with all-ones mask

A vmv.v.v shares the same encoding as a vmerge that isn't masked, so we can
also fold it into its operands if we treat it as a vmerge with an all-ones
mask.  We take care here not to actually transform the existing vmv into a
vmerge, otherwise things like True.hasOneUse() become inaccurate. Instead this
just returns an equivalent list of operands.
This is an alternative to D153351.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D155101