Implement loop invariant code motion for check instructions.

runtime/vm/compiler.cc

Index: runtime/vm/compiler.cc
===================================================================
--- runtime/vm/compiler.cc	(revision 11934)
+++ runtime/vm/compiler.cc	(working copy)
@@ -36,6 +36,7 @@
 DEFINE_FLAG(bool, trace_bailout, false, "Print bailout from ssa compiler.");
 DEFINE_FLAG(bool, trace_compiler, false, "Trace compiler operations.");
 DEFINE_FLAG(bool, cse, true, "Do common subexpression elimination.");
+DEFINE_FLAG(bool, licm, true, "Do loop invariant code motion.");
 DEFINE_FLAG(int, deoptimization_counter_threshold, 5,
     "How many times we allow deoptimization before we disallow"
     " certain optimizations");
@@ -213,6 +214,9 @@
           flow_graph->ComputeUseLists();
           DominatorBasedCSE::Optimize(flow_graph->graph_entry());
         }
+        if (FLAG_licm) {
+          LICM::Optimize(flow_graph);
+        }
 
         // Perform register allocation on the SSA graph.
         FlowGraphAllocator allocator(*flow_graph);