[turbofan] Rework handling of loop exits in loop peeling.

src/compiler/loop-analysis.h

Index: src/compiler/loop-analysis.h
diff --git a/src/compiler/loop-analysis.h b/src/compiler/loop-analysis.h
index 71f946113d41dfb725abfeca98ac5c880033042e..77c2a95928c5e6083a61d11cbe3f3450439ba9c1 100644
--- a/src/compiler/loop-analysis.h
+++ b/src/compiler/loop-analysis.h
@@ -14,6 +14,9 @@ namespace v8 {
 namespace internal {
 namespace compiler {
 
+// TODO(titzer): don't assume entry edges have a particular index.
+static const int kAssumedLoopEntryIndex = 0;  // assume loops are entered here.
+
 class LoopFinderImpl;
 
 typedef base::iterator_range<Node**> NodeRange;
@@ -140,7 +143,6 @@ class LoopTree : public ZoneObject {
   ZoneVector<Node*> loop_nodes_;
 };
 
-
 class LoopFinder {
  public:
   // Build a loop tree for the entire graph.