Don't form a WeakVH around the sentinel node in the instructions BB

Don't form a WeakVH around the sentinel node in the instructions BB
list. This is a bad idea. ;] I'm hopeful this is the bug that's showing
up with the MSVC bots, but we'll see.

It is definitely unnecessary. InstSimplify won't do anything to
a terminator instruction, we don't need to even include it in the
iteration range. We can also skip the now dead terminator check,
although I've made it an assert to help document that this is an
important invariant.

I'm still a bit queasy about this because there is an implicit
assumption that the terminator instruction cannot be RAUW'ed by the
simplification code. While that appears to be true at the moment, I see
no guarantee that would ensure it remains true in the future. I'm
looking at the cleanest way to solve that...

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@153399 91177308-0d34-0410-b5e6-96231b3b80d8