[llvm] Add isa_and_nonnull

Summary:
Add new ``isa_and_nonnull<>`` operator that works just like
the ``isa<>`` operator, except that it allows for a null pointer as an
argument (which it then returns false).

Reviewers: lattner, aaron.ballman, greened

Reviewed By: lattner

Subscribers: hubert.reinterpretcast, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60291

llvm-svn: 357761