Just because we cannot completely eliminate all uses of a global, we can

Just because we cannot completely eliminate all uses of a global, we can
still optimize away all of the indirect calls and loads, etc from it.
This turns code like this:

  if (G != 0)
    G();

into
   if (G != 0)
     ActualCallee();

This triggers a couple of times in gcc and libstdc++.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@16901 91177308-0d34-0410-b5e6-96231b3b80d8