[RISCV] Match strided load via DAG combine (#66800)

This change matches a masked.stride.load from a mgather node whose index
operand is a strided sequence. We can reuse the VID matching from
build_vector lowering for this purpose.

Note that this duplicates the matching done at IR by
RISCVGatherScatterLowering.cpp. Now that we can widen gathers to a wider
SEW, I don't see a good way to remove this duplication. The only obvious
alternative is to move thw widening transform to IR, but that's a no-go
as I want other DAGs to run first. I think we should just live with the
duplication - particularly since the reuse is isSimpleVIDSequence means
the duplication is somewhat minimal.