Elide unnecessary context reload in generated stubs.

src/lithium-allocator.cc

Index: src/lithium-allocator.cc
diff --git a/src/lithium-allocator.cc b/src/lithium-allocator.cc
index b23c86766a6fb71e86927aebefdd1b2034aab359..dcaa91e8662860ea21619ff27fa93e66a76c1e4c 100644
--- a/src/lithium-allocator.cc
+++ b/src/lithium-allocator.cc
@@ -830,6 +830,7 @@ void LAllocator::MeetConstraintsBetween(LInstruction* first,
   // Handle fixed input operands of second instruction.
   if (second != NULL) {
     for (UseIterator it(second); !it.Done(); it.Advance()) {
+      if (it.Current() == NULL) continue;
       LUnallocated* cur_input = LUnallocated::cast(it.Current());
       if (cur_input->HasFixedPolicy()) {
         LUnallocated* input_copy = cur_input->CopyUnconstrained(zone());
@@ -973,6 +974,7 @@ void LAllocator::ProcessInstructions(HBasicBlock* block, BitVector* live) {
 
         for (UseIterator it(instr); !it.Done(); it.Advance()) {
           LOperand* input = it.Current();
+          if (input == NULL) continue;
 
           LifetimePosition use_pos;
           if (input->IsUnallocated() &&