Add support for adding an external and a tagged pointer

src/x64/lithium-x64.cc

Index: src/x64/lithium-x64.cc
diff --git a/src/x64/lithium-x64.cc b/src/x64/lithium-x64.cc
index f1710bd561036c290ba54f0c581f0ce876b4dc62..2d9bd9530d3b8cec9f3427bd57ea6cb17428e077 100644
--- a/src/x64/lithium-x64.cc
+++ b/src/x64/lithium-x64.cc
@@ -1620,7 +1620,10 @@ LInstruction* LChunkBuilder::DoAdd(HAdd* instr) {
     return result;
   } else if (instr->representation().IsExternal()) {
     DCHECK(instr->left()->representation().IsExternal());
-    DCHECK(instr->right()->representation().IsInteger32());
+    DCHECK((instr->external_add_type() == AddOfExternalAndInt32 &&
+            instr->right()->representation().IsInteger32()) ||
+           (instr->external_add_type() == AddOfExternalAndTagged &&
+            instr->right()->representation().IsTagged()));
     DCHECK(!instr->CheckFlag(HValue::kCanOverflow));
     bool use_lea = LAddI::UseLea(instr);
     LOperand* left = UseRegisterAtStart(instr->left());