[LV] Avoid emitting trivially dead instructions

Some instructions from the original loop, when vectorized, can become trivially
dead. This happens because of the way we structure the new loop. For example,
we create new induction variables and induction variable "steps" in the new
loop. Thus, when we go to vectorize the original induction variable update, it
may no longer be needed due to the instructions we've already created. This
patch prevents us from creating these redundant instructions. This reduces code
size before simplification and allows greater flexibility in code generation
since we have fewer unnecessary instruction uses.

Differential Revision: https://reviews.llvm.org/D25631

llvm-svn: 284631