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 2d72687d31c77c25cb6a39869cbcf0731e635bc0..bd46dd8f58c73d03d10d19e59241c1c2cad17d99 100644
--- a/src/arm/lithium-arm.cc
+++ b/src/arm/lithium-arm.cc
@@ -1675,8 +1675,7 @@ LInstruction* LChunkBuilder::DoAdd(HAdd* instr) {
     }
     return result;
   } else if (instr->representation().IsExternal()) {
-    DCHECK(instr->left()->representation().IsExternal());
-    DCHECK(instr->right()->representation().IsInteger32());
+    DCHECK(instr->IsConsistentExternalRepresentation());
     DCHECK(!instr->CheckFlag(HValue::kCanOverflow));
     LOperand* left = UseRegisterAtStart(instr->left());
     LOperand* right = UseOrConstantAtStart(instr->right());