[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

llvm/lib/Transforms/IPO/PassManagerBuilder.cpp

@@ -234,6 +234,8 @@ void PassManagerBuilder::addFunctionSimplificationPasses(
   else
     MPM.add(createScalarReplAggregatesPass(-1, false));
   MPM.add(createEarlyCSEPass());              // Catch trivial redundancies
+  // Speculative execution if the target has divergent branches; otherwise nop.
+  MPM.add(createSpeculativeExecutionIfHasBranchDivergencePass());
   MPM.add(createJumpThreadingPass());         // Thread jumps.
   MPM.add(createCorrelatedValuePropagationPass()); // Propagate conditionals
   MPM.add(createCFGSimplificationPass());     // Merge & remove BBs