Elide unnecessary context reload in generated stubs.

src/lithium-allocator-inl.h

Index: src/lithium-allocator-inl.h
diff --git a/src/lithium-allocator-inl.h b/src/lithium-allocator-inl.h
index 8f660ce0e00373bdd5521b183527e972fc68c75c..1f59cb47783fac74a42fb94502e97eecc352ab54 100644
--- a/src/lithium-allocator-inl.h
+++ b/src/lithium-allocator-inl.h
@@ -99,6 +99,7 @@ bool InputIterator::Done() { return current_ >= limit_; }
 
 LOperand* InputIterator::Current() {
   ASSERT(!Done());
+  ASSERT(instr_->InputAt(current_) != NULL);
   return instr_->InputAt(current_);
 }
 
@@ -110,7 +111,9 @@ void InputIterator::Advance() {
 
 
 void InputIterator::SkipUninteresting() {
-  while (current_ < limit_ && instr_->InputAt(current_)->IsConstantOperand()) {
+  while (current_ < limit_) {
+    LOperand* current = instr_->InputAt(current_);
+    if (current != NULL && !current->IsConstantOperand()) break;
     ++current_;
   }
 }
@@ -127,9 +130,11 @@ bool UseIterator::Done() {
 
 LOperand* UseIterator::Current() {
   ASSERT(!Done());
-  return input_iterator_.Done()
+  LOperand* result = input_iterator_.Done()
       ? env_iterator_.Current()
       : input_iterator_.Current();
+  ASSERT(result != NULL);
+  return result;
 }