Introduce new subclasses of VPWidenMemoryRecipe for VP
(vector-predicated) loads and stores to address multiple TODOs from
#76172

Note that the introduction of the new recipes also improves code-gen for
VP gather/scatters by removing the redundant header mask. With the new
approach, it is not sufficient to look at users of the widened canonical
IV to find all uses of the header mask.

In some cases, a widened IV is used instead of separately widening the
canonical IV. To handle those cases, iterate over all recipes in the
vector loop region to make sure all widened memory recipes are
processed.

Depends on #87411.