Fix clobbered register when setting this_function variable.

src/mips64/full-codegen-mips64.cc

Index: src/mips64/full-codegen-mips64.cc
diff --git a/src/mips64/full-codegen-mips64.cc b/src/mips64/full-codegen-mips64.cc
index 572b5b3a77569a637834ccfff86a49b553da7e04..34e37b37bf7edb85249e2816736e3323af534ea5 100644
--- a/src/mips64/full-codegen-mips64.cc
+++ b/src/mips64/full-codegen-mips64.cc
@@ -249,6 +249,10 @@ 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) {
+      __ ld(a1, MemOperand(fp, JavaScriptFrameConstants::kFunctionOffset));
+      function_in_register = true;
+    }
     SetVar(this_function_var, a1, a2, a3);
   }