[LV] Apply sink-after & interleave-groups as VPlan transformations (NFCI)

This recommits 11ed1c02 (reverted in
9f08ce0d for failing an assert) with a fix:
tryToWidenMemory() now first checks if the widening decision is to interleave,
thus maintaining previous behavior where tryToInterleaveMemory() was called
first, giving priority to interleave decisions over widening/scalarization. This
commit adds the test case that exposed this bug as a LIT.