Extend the check to detect patterns like 'ptr.get() == nullptr'

Summary:
Extend the check to detect patterns like 'ptr.get() == nullptr'
It detects == and != when any argument is a ptr.get() and the other is a
nullptr.
Only supports standard smart pointer types std::unique_ptr and std::shared_ptr.
Does not support the case 'ptr.get() == other.get()' yet.

Reviewers: djasper

CC: cfe-commits, alexfh

Differential Revision: http://llvm-reviews.chandlerc.com/D3294