[LV] 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@204184 91177308-0d34-0410-b5e6-96231b3b80d8