[LV] Remove collectTriviallyDeadInstructions, already handled by VP DCE.

Now that removeDeadRecipes can remove most dead recipes across a whole
VPlan, there is no need to first collect some dead instructions.
Instead removeDeadRecipes can simply clean them up.

Depends D127580.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D128408