Fix HConstant's hash function for smis on x64.

src/hydrogen-instructions.h

Index: src/hydrogen-instructions.h
diff --git a/src/hydrogen-instructions.h b/src/hydrogen-instructions.h
index 011529d6cdfd94f85f2029b93e93ee07333d6e19..1ad3e0cb4834467a9743312d59993cc91d980880 100644
--- a/src/hydrogen-instructions.h
+++ b/src/hydrogen-instructions.h
@@ -2457,7 +2457,11 @@ class HConstant: public HTemplateInstruction<0> {
 
   virtual intptr_t Hashcode() {
     ASSERT(!HEAP->allow_allocation(false));
-    return reinterpret_cast<intptr_t>(*handle());
+    intptr_t hash = reinterpret_cast<intptr_t>(*handle());
+    // Prevent smis from having fewer hash values when truncated to
+    // the least significant bits.
+    const int kShiftSize = kSmiShiftSize + kSmiTagSize;
+    return hash ^ (hash >> kShiftSize);
   }
 
 #ifdef DEBUG