compiler: Don't make temporaries for constant operands.

Fixes golang/go#12616.
    
    Reviewed-on: https://go-review.googlesource.com/14645

From-SVN: r228578

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-651e71a729e5dcbd9dc14c1b59b6eff05bfe3d26
+a4bcd319d98ddc52b3e7d16ec87d92aad868ab05
 
 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

@@ -5200,13 +5200,13 @@ Binary_expression::do_flatten(Gogo* gogo, Named_object*,
       || (is_idiv_op
 	  && (gogo->check_divide_by_zero() || gogo->check_divide_overflow())))
     {
-      if (!this->left_->is_variable())
+      if (!this->left_->is_variable() && !this->left_->is_constant())
         {
           temp = Statement::make_temporary(NULL, this->left_, loc);
           inserter->insert(temp);
           this->left_ = Expression::make_temporary_reference(temp, loc);
         }
-      if (!this->right_->is_variable())
+      if (!this->right_->is_variable() && !this->right_->is_constant())
         {
           temp =
               Statement::make_temporary(NULL, this->right_, loc);