[LV] The actual change I intended to commit in r204148. Sorry for the
noise.

Original commit log:
Replace some dead code with an assert. When I first ported this pass
from a loop pass to a function pass I did so in the naive, recursive
way. It doesn't actually work, we need a worklist instead. When
I switched to the worklist I didn't delete the naive recursion. That
recursion was also buggy because it was dead and never really exercised.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204187 91177308-0d34-0410-b5e6-96231b3b80d8