Just comparing constant trip counts causes LV to miss cases where the
vector loop body only executes once.
The motivation for this is to remove the need for unrolling to remove
vector loop back-edges, if the body only executes once.
It requires using non-recursive SCEV reasoning, as at this stage the CFG
is incomplete and only reasoning based on existing expression can be
used. In particular, more complex strategies like proving via induction
cannot be used, as LI/DT may be out of date.
Alternatively the result of the check could be computed earlier.