If a VL is zero then it's known to be less than or equal to every other
VL.
This looks weird on its own since a VL of zero isn't that common. The
test diffs come from a type being split resulting in a VP intrinsic's
EVL being zero.
The motivation for this is to split off part of an upcoming patch I plan
on submitting for RISCVVLOptimizer, which generalizes it to handle
recurrences, and needs to reason about an initial state of demanded VLs
set to zero.
bc2004c2e4