Fix a horrible infloop in value tracking in the face of dead code.

Amazingly, we just never triggered this without:
1) Moving code around for MetadataTracking so that a certain *different*
   amount of inlining occurs in the per-TU compile step.
2) Then you LTO opt or clang with a bootstrap, and get inlining, loop
   opts, and GVN line up everything *just* right.

I don't really know how we didn't hit this before. We really need to be
fuzz testing stuff, it shouldn't be hard to trigger. I'm working on
crafting a reduced nice test case, and will submit that when I have it,
but I want to get LTO build bots going again.

llvm-svn: 256735