Teach the inliner how to preserve musttail invariants

The interesting case is what happens when you inline a musttail call
through a musttail call site.  In this case, we can't break perfect
forwarding or allow any stack growth.

Instead of merging control flow from the inlined return instruction
after a musttail call into the body of the caller, leave the inlined
return instruction in the caller so that the musttail call stays in the
tail position.

More work is required in http://reviews.llvm.org/D3630 to handle the
case where the inlined function has dynamic allocas or byval arguments.

Reviewers: chandlerc

Differential Revision: http://reviews.llvm.org/D3491