Add support for adding an external and a tagged pointer

src/arm64/lithium-arm64.cc

Index: src/arm64/lithium-arm64.cc
diff --git a/src/arm64/lithium-arm64.cc b/src/arm64/lithium-arm64.cc
index 920ea3e6c5d14b158f38f3cebc4bee44d9527782..19523645ccfb45ebb557426646653e562edf45cd 100644
--- a/src/arm64/lithium-arm64.cc
+++ b/src/arm64/lithium-arm64.cc
@@ -902,7 +902,10 @@ 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));
     LOperand* left = UseRegisterAtStart(instr->left());
     LOperand* right = UseRegisterOrConstantAtStart(instr->right());