[LV] Fix crash for reverse interleaved loads with gap under fold-tail.

This patch fixes the crash found by PR51614:
whenever doing tail folding, interleave groups must be considered under mask.

Another fix D108900 follows for targets that support masked loads and stores:
when *deciding* to vectorize with masked interleave groups, check if the access
is reverse - which is currently not supported; rather than (only) asserting when
computing cost and generating code.

Differential Revision: https://reviews.llvm.org/D108891