Use an allocation site scratchpad to speed up allocaton site processing during gc.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 5a2d5595fc9cf2fd12bf166494b22a0136db66d0..47b2e31542d43ea937b575c0f0be4043841eebcd 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -8138,7 +8138,9 @@ class AllocationSite: public Struct {
   class UnusedBits:             public BitField<int,          15, 14> {};
   class DoNotInlineBit:         public BitField<bool,         29,  1> {};
 
-  inline void IncrementMementoFoundCount();
+  // Increments the mementos found counter and returns true when the first
+  // memento was found for a given allocation site.
+  inline bool IncrementMementoFoundCount();
 
   inline void IncrementMementoCreateCount();