[VPlan] Model masked assumes as replicate recipes, drop them (NFCI).

Replace ConditionalAssume set by treating conditional assumes like other
predicated instructions (i.e. create a VPReplicateRecipe with a mask)
and later remove any assume recipes with masks during VPlan cleanup.

This reduces coupling of VPlan construction and Legal by removing a
shared set between the 2 and results in a cleaner code structure
overall.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D157034