[RISCV] Combine a gather to a larger element type (#66694)

If we have a gather load whose indices correspond to valid offsets for a
gather with element type twice that our source, we can reduce the number
of indices and perform the operation at the larger element type.

This is generally profitable since we half VL - and these operations are
linear in VL. This may require some additional VL/VTYPE toggles, but
this appears to be worthwhile on the whole.