Elide unnecessary context reload in generated stubs.

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 574970c6994e701cab44a590ac58d0f6aa121479..11c8a32d2693d6f28ad39271c84eb9715e20c48e 100644
--- a/src/x64/lithium-x64.cc
+++ b/src/x64/lithium-x64.cc
@@ -1006,7 +1006,14 @@ LInstruction* LChunkBuilder::DoThisFunction(HThisFunction* instr) {
 
 
 LInstruction* LChunkBuilder::DoContext(HContext* instr) {
-  return instr->HasNoUses() ? NULL : DefineAsRegister(new(zone()) LContext);
+  // If there is a non-return use, the context must be allocated in a register.
+  for (HUseIterator it(instr->uses()); !it.Done(); it.Advance()) {
+    if (!it.value()->IsReturn()) {
+      return DefineAsRegister(new(zone()) LContext);
+    }
+  }
+
+  return NULL;
 }