Fix clobbered register when setting this_function variable.

src/arm64/full-codegen-arm64.cc

Index: src/arm64/full-codegen-arm64.cc
diff --git a/src/arm64/full-codegen-arm64.cc b/src/arm64/full-codegen-arm64.cc
index 454bf03ddcbd2862c1ea02a5edc63437c020e3fa..d35f28479a932772b43fc494ea7efbdc2b31ad9d 100644
--- a/src/arm64/full-codegen-arm64.cc
+++ b/src/arm64/full-codegen-arm64.cc
@@ -244,6 +244,10 @@ 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_x1) {
+      __ Ldr(x1, MemOperand(fp, JavaScriptFrameConstants::kFunctionOffset));
+      function_in_register_x1 = true;
+    }
     SetVar(this_function_var, x1, x0, x2);
   }