[Devirtualization] MemDep returns non-local !invariant.group dependencies

Summary:
Memory Dependence Analysis was limited to return only local dependencies
for invariant.group handling. Now it returns NonLocal when it finds it
and then by asking getNonLocalPointerDependency we get found dep.

Thanks to this we are able to devirtualize loops!

    void indirect(A &a, int n) {
      for (int i = 0 ; i < n; i++)
        a.foo();

    }
    void test(int n) {
      A a;
      indirect(a);
    }

After inlining a.foo() will be changed to direct call, even if foo and A::A()
is external (but only if vtable definition is be available).

Reviewers: nlewycky, dberlin, chandlerc, rsmith

Subscribers: mehdi_amini, davide, llvm-commits

Differential Revision: https://reviews.llvm.org/D28137

llvm-svn: 291762