- Use the "Fast" flag instead of "OptimizeForSize" to determine whether to emit

- Use the "Fast" flag instead of "OptimizeForSize" to determine whether to emit
  a DBG_LABEL or not. We want to fall back to the original way of emitting debug
  info when we're in -O0/-fast mode.
- Add plumbing in to pass the "Fast" flag to places that need it.
- XFAIL DebugInfo/deaddebuglabel.ll. This is finding 11 labels instead of 8. I
  need to investigate still.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@65367 91177308-0d34-0410-b5e6-96231b3b80d8