Elide unnecessary context reload in generated stubs.

src/lithium.h

Index: src/lithium.h
diff --git a/src/lithium.h b/src/lithium.h
index cc2cde015a4539dbf2b2aa77440ccba8a78e88e1..ea61ff5ca8acb5461d3b7b7d12f136d5dd102406 100644
--- a/src/lithium.h
+++ b/src/lithium.h
@@ -581,6 +581,7 @@ class ShallowIterator BASE_EMBEDDED {
 
   LOperand* Current() {
     ASSERT(!Done());
+    ASSERT(env_->values()->at(current_) != NULL);
     return env_->values()->at(current_);
   }
 
@@ -622,6 +623,7 @@ class DeepIterator BASE_EMBEDDED {
 
   LOperand* Current() {
     ASSERT(!current_iterator_.Done());
+    ASSERT(current_iterator_.Current() != NULL);
     return current_iterator_.Current();
   }