Introduce a @llvm.experimental.guard intrinsic

Summary:
As discussed on llvm-dev[1].

This change adds the basic boilerplate code around having this intrinsic
in LLVM:

 - Changes in Intrinsics.td, and the IR Verifier
 - A lowering pass to lower @llvm.experimental.guard to normal
   control flow
 - Inliner support

[1]: http://lists.llvm.org/pipermail/llvm-dev/2016-February/095523.html

Reviewers: reames, atrick, chandlerc, rnk, JosephTremoulet, echristo

Subscribers: mcrosier, llvm-commits

Differential Revision: http://reviews.llvm.org/D18527

llvm-svn: 264976