Add support for adding an external and a tagged pointer

src/ppc/lithium-ppc.cc

Index: src/ppc/lithium-ppc.cc
diff --git a/src/ppc/lithium-ppc.cc b/src/ppc/lithium-ppc.cc
index 296f0ac364fce880be25ff33c5e25647eab2b13f..15782e1075848606588755facc72e4608f430b7c 100644
--- a/src/ppc/lithium-ppc.cc
+++ b/src/ppc/lithium-ppc.cc
@@ -1645,8 +1645,7 @@ 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());