Add support for adding an external and a tagged pointer

src/mips64/lithium-mips64.cc

Index: src/mips64/lithium-mips64.cc
diff --git a/src/mips64/lithium-mips64.cc b/src/mips64/lithium-mips64.cc
index 97c2d2ef753a7c5312715a6578cbb1d6d93c6d3c..7d06ec747fda8e2c58c23c2878b8e2220c7e4867 100644
--- a/src/mips64/lithium-mips64.cc
+++ b/src/mips64/lithium-mips64.cc
@@ -1630,8 +1630,7 @@ 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 = UseRegisterOrConstantAtStart(instr->right());