Fix clobbered register when setting this_function variable.

src/x64/full-codegen-x64.cc

Index: src/x64/full-codegen-x64.cc
diff --git a/src/x64/full-codegen-x64.cc b/src/x64/full-codegen-x64.cc
index 0b19edd683084bd92d7f67ebdff57ec6f3d43d12..879d670a0b24431e408c330c313bc6ef9ebba541 100644
--- a/src/x64/full-codegen-x64.cc
+++ b/src/x64/full-codegen-x64.cc
@@ -235,6 +235,10 @@ void FullCodeGenerator::Generate() {
   Variable* this_function_var = scope()->this_function_var();
   if (this_function_var != nullptr) {
     Comment cmnt(masm_, "[ This function");
+    if (!function_in_register) {
+      __ movp(rdi, Operand(rbp, JavaScriptFrameConstants::kFunctionOffset));
+      // The write barrier clobbers register again, keep is marked as such.
+    }
     SetVar(this_function_var, rdi, rbx, rdx);
   }