Fix three bugs:

 1. Calls to external global VARIABLES should not be treated as a call to an
    external function
 2. Efficiently deleting an element from a vector by using std::swap with
    the back, then pop_back is NOT a good way to keep the vector sorted.
 3. Our hope of having stuff get deleted by making them redundant just won't
    work.  In particular, if we have three calls in sequence that should be
    merged: A, B, C   first we unify B into A.  To be sure that they appeared
    identical (so B would be erased) we set B = A.  On the next step, we
    unified C into A and set C = A.  Unfortunately, this is no guarantee that
    C = B, so we would fail to delete the dead call.  Switch to a more
    explicit scheme.

llvm-svn: 17357