[BPI] Consider deoptimize calls as "unreachable"

Summary:
Calls to @llvm.experimental.deoptimize are expected to "never execute",
so optimize them as such.

Reviewers: chandlerc

Subscribers: junbuml, mcrosier, llvm-commits

Differential Revision: http://reviews.llvm.org/D19095

llvm-svn: 266654