Fixed printing of LInstructions in the presence of optional arguments

src/x64/lithium-x64.cc

Index: src/x64/lithium-x64.cc
diff --git a/src/x64/lithium-x64.cc b/src/x64/lithium-x64.cc
index 070f1a896f184fed5b22765fb48324ac779e837c..85eeee1ac0edacaa620abedaabc8bddd0af2e6b7 100644
--- a/src/x64/lithium-x64.cc
+++ b/src/x64/lithium-x64.cc
@@ -114,7 +114,11 @@ void LInstruction::PrintDataTo(StringStream* stream) {
   stream->Add("= ");
   for (int i = 0; i < InputCount(); i++) {
     if (i > 0) stream->Add(" ");
-    InputAt(i)->PrintTo(stream);
+    if (InputAt(i) == NULL) {
+      stream->Add("NULL");
+    } else {
+      InputAt(i)->PrintTo(stream);
+    }
   }
 }