[turbofan] Run value numbering after SimplifiedLowering.

Running ValueNumberingReducer together with ChangeLowering causes
trouble with the current GraphReducer and makes GVN ineffective, so
running it before we get to change lowering works around that well-known
(phase ordering) problem for now.

R=jarin@chromium.org

Review URL: https://codereview.chromium.org/660733003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24656 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/compiler/pipeline.cc

@@ -316,6 +316,12 @@ Handle<Code> Pipeline::GenerateCode() {
                                      SourcePosition::Unknown());
       SimplifiedLowering lowering(&jsgraph);
       lowering.LowerAllNodes();
+      ValueNumberingReducer vn_reducer(zone());
+      SimplifiedOperatorReducer simple_reducer(&jsgraph);
+      GraphReducer graph_reducer(&graph);
+      graph_reducer.AddReducer(&simple_reducer);
+      graph_reducer.AddReducer(&vn_reducer);
+      graph_reducer.ReduceGraph();
 
       VerifyAndPrintGraph(&graph, "Lowered simplified");
     }
@@ -327,13 +333,11 @@ Handle<Code> Pipeline::GenerateCode() {
                                      SourcePosition::Unknown());
       Linkage linkage(info());
       ValueNumberingReducer vn_reducer(zone());
-      SimplifiedOperatorReducer simple_reducer(&jsgraph);
       ChangeLowering lowering(&jsgraph, &linkage);
       MachineOperatorReducer mach_reducer(&jsgraph);
       GraphReducer graph_reducer(&graph);
       // TODO(titzer): Figure out if we should run all reducers at once here.
       graph_reducer.AddReducer(&vn_reducer);
-      graph_reducer.AddReducer(&simple_reducer);
       graph_reducer.AddReducer(&lowering);
       graph_reducer.AddReducer(&mach_reducer);
       graph_reducer.ReduceGraph();