Make getLastArgNoClaim work for up to 4 arguments.

Summary:
This is needed for http://reviews.llvm.org/D8507
I have no idea what stand-alone tests could be done, if needed.

Reviewers: Bigcheese, craig.topper, samsonov

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D8508

llvm-svn: 232859