Add support for adding an external and a tagged pointer

src/arm/lithium-arm.cc

Index: src/arm/lithium-arm.cc
diff --git a/src/arm/lithium-arm.cc b/src/arm/lithium-arm.cc
index 55e501762c4fece70a9264ac24068dcad39dc37b..138939c06ef477e4126670e8ce0c984456cfefa1 100644
--- a/src/arm/lithium-arm.cc
+++ b/src/arm/lithium-arm.cc
@@ -1662,7 +1662,8 @@ LInstruction* LChunkBuilder::DoAdd(HAdd* instr) {
     return result;
   } else if (instr->representation().IsExternal()) {
     DCHECK(instr->left()->representation().IsExternal());
-    DCHECK(instr->right()->representation().IsInteger32());
+    DCHECK(instr->right()->representation().IsInteger32() ||
+           instr->right()->representation().IsTagged());
     DCHECK(!instr->CheckFlag(HValue::kCanOverflow));
     LOperand* left = UseRegisterAtStart(instr->left());
     LOperand* right = UseOrConstantAtStart(instr->right());