Add support for adding an external and a tagged pointer

src/mips/lithium-mips.cc

Index: src/mips/lithium-mips.cc
diff --git a/src/mips/lithium-mips.cc b/src/mips/lithium-mips.cc
index 3f0e7bfd27a0cffabfcb8f1564efcf2418f1529f..733866572fb97c585e098747b38a2e522fe5151a 100644
--- a/src/mips/lithium-mips.cc
+++ b/src/mips/lithium-mips.cc
@@ -1622,8 +1622,7 @@ 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());