[VPlan] Ignore incoming values with constant false mask. (#89384)

Ignore incoming values with constant false masks when trying to simplify
VPBlendRecipes.

As a follow-on optimization, we should also be able to drop all incoming
values with false masks by creating a new VPBlendRecipe with those
operands dropped.

PR: https://github.com/llvm/llvm-project/pull/89384