Lazy removal of dead HValues in GVN from use lists.

src/hydrogen-instructions.h

Index: src/hydrogen-instructions.h
diff --git a/src/hydrogen-instructions.h b/src/hydrogen-instructions.h
index 4d7b7baa880e81fe3ee780dd20de3171006949bd..4a01085043960f0881b3a28edee51c5c606b4ced 100644
--- a/src/hydrogen-instructions.h
+++ b/src/hydrogen-instructions.h
@@ -448,7 +448,7 @@ class HUseListNode: public ZoneObject {
       : tail_(tail), value_(value), index_(index) {
   }
 
-  HUseListNode* tail() const { return tail_; }
+  HUseListNode* tail();
   HValue* value() const { return value_; }
   int index() const { return index_; }
 
@@ -530,7 +530,8 @@ class HValue: public ZoneObject {
     kDeoptimizeOnUndefined,
     kIsArguments,
     kTruncatingToInt32,
-    kLastFlag = kTruncatingToInt32
+    kIsDead,
+    kLastFlag = kIsDead
   };
 
   STATIC_ASSERT(kLastFlag < kBitsPerInt);