[RISCV][InsertVSETVLI] Move vmv.s.x SEW check into getDemandedBits. NFC

This patch restructures the logic that checks if vmv.s.x's SEW can be
expanded into getDemandedBits, so that it can be shared by both the
top-to-bottom and bottom-to-top passes.

It adds a third option for SEW in DemandedFields, that's weaker than
demanded but stronger than not demanded, that states that it the new SEW
must be greater than or equal to the current SEW.

Note that we now need to take care of the order of operands in
areCompatibleVTYPEs as the relation is no longer commutative.

A later patch will remove the gating on the bottom-to-top pass
(dolocalPostpass) and another one will relax the demands on the tail
policy further.