compiler: Don't make temporaries for constant operands.

Fixes golang/go#12616.

Change-Id: I07e02744b7af94f5820ddbd3360d33e2573ce49b
Reviewed-on: https://go-review.googlesource.com/14645
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: Chris Manghane

go/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);