compiler: fix double evaluation with interface field expression

    
    In Interface_field_reference_expression, the interface expression
    is used in two places, so a temporary variable is used. Previously,
    we used a Set_and_use_temporary_expression, which, when evaluated
    twice, causes double evaluation of the underlying expression. Fix
    by setting the temporary once and use Temporary_reference_expression
    instead.
    
    Fixes golang/go#26248.
    
    Reviewed-on: https://go-review.googlesource.com/122756

From-SVN: r262533

gcc/go/gofrontend/MERGE

-94738979a3422e845acf358a766aabf8b9275d43
+8ad67a72a4fa59efffc891e73ecf10020e3c565d
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/expressions.cc

@@ -11886,10 +11886,9 @@ Interface_field_reference_expression::do_flatten(Gogo*, Named_object*,
   if (!this->expr_->is_variable())
     {
       Temporary_statement* temp =
-	Statement::make_temporary(this->expr_->type(), NULL, this->location());
+	Statement::make_temporary(NULL, this->expr_, this->location());
       inserter->insert(temp);
-      this->expr_ = Expression::make_set_and_use_temporary(temp, this->expr_,
-							   this->location());
+      this->expr_ = Expression::make_temporary_reference(temp, this->location());
     }
   return this;
 }