[PM] Add a SpeculativeExecution pass for targets with divergent branches.

Summary:
This IR pass is helpful for GPUs, and other targets with divergent
branches.  It's a nop on targets without divergent branches.

Reviewers: chandlerc

Subscribers: llvm-commits, jingyue, rnk, joker.eph, tra

Differential Revision: http://reviews.llvm.org/D18626

llvm-svn: 266399