[RISCV][InsertVSETVLI] Generalize scalar move rule for when AVL is unchanged

By definition, the AVL of the scalar move is equally zero to the prior AVL if they are the same value.  This generalizes the existing code to the case where the scalar move has a register AVL which is unknown, but unchanged from the preceeding instruction.

This doesn't cause any interesting diffs on its own, but another patch makes this case much more common.  Split off to reduce a future diff.