Finally fix (the right way) the problem where functions like this:

Finally fix (the right way) the problem where functions like this:

void foo() {
  G = 1;
}

would have an empty DSGraph even though G (a global) is directly used
in the function.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@20619 91177308-0d34-0410-b5e6-96231b3b80d8