Revert r28050 due to crashes with --optimization-counter-threshold=5.

runtime/vm/compiler.cc

Index: runtime/vm/compiler.cc
===================================================================
--- runtime/vm/compiler.cc	(revision 28051)
+++ runtime/vm/compiler.cc	(working copy)
@@ -452,8 +452,7 @@
 
         if (FLAG_constant_propagation) {
           // Constant propagation can use information from range analysis to
-          // find unreachable branch targets and eliminate branches that have
-          // the same true- and false-target.
+          // find unreachable branch targets.
           ConstantPropagator::OptimizeBranches(flow_graph);
           DEBUG_ASSERT(flow_graph->VerifyUseLists());
         }