[Speculation] Add a SpeculativeExecution mode where the pass does nothing...

[Speculation] Add a SpeculativeExecution mode where the pass does nothing unless TTI::hasBranchDivergence() is true.

Summary:
This lets us add this pass to the IR pass manager unconditionally; it
will simply not do anything on targets without branch divergence.

Reviewers: tra

Subscribers: llvm-commits, jingyue, rnk, chandlerc

Differential Revision: http://reviews.llvm.org/D18625

llvm-svn: 266398