Replace the core comparison login in merge functions. We can now merge

vector<>::push_back() in:

  int foo(vector<int> &a, vector<unsigned> &b) {
    a.push_back(10);
    b.push_back(11);
  }

to two calls to the same push_back function, or fold away the two copies of
push_back() in:

  struct T { int; };
  struct S { char; };
  vector<T*> t;
  vector<S*> s;
  void f(T *x) { t.push_back(x); }
  void g(S *x) { s.push_back(x); }

but leave f() and g() separate, since they refer to two different global
variables.

llvm-svn: 103698