Add support for adding an external and a tagged pointer

src/x87/lithium-x87.cc

Index: src/x87/lithium-x87.cc
diff --git a/src/x87/lithium-x87.cc b/src/x87/lithium-x87.cc
index a57aa915767b132ff53121b0309190a0423055f8..740675c99f9558103a411e1ecae51b794e3fe6a0 100644
--- a/src/x87/lithium-x87.cc
+++ b/src/x87/lithium-x87.cc
@@ -1626,7 +1626,8 @@ LInstruction* LChunkBuilder::DoAdd(HAdd* instr) {
     return DoArithmeticD(Token::ADD, instr);
   } 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));
     bool use_lea = LAddI::UseLea(instr);
     LOperand* left = UseRegisterAtStart(instr->left());