[RISCV] Use an enum for demanded LMUL in RISCVInsertVSETVLI. NFC (#92513)

In needVSETVLI used by the forward insertion pass, we have some rules
where we can relax the demanded fields for slides and splats when VL=1.

However this only works if we don't increase LMUL to anything > M1
otherwise we would end up clobbering random registers.

Rather than check the VSETVLIInfo we're transitioning to, store this
information in DemandedFields and have isCompatible check it.

That way an upcoming patch can share these VL=1 rules with
RISCVCoalesceVSETVLI, which uses isCompatible and not needVSETVLI.