compiler: Report errors for non-integral shift counts.

    
    Fixes golang/go#12618.
    
    Reviewed-on: https://go-review.googlesource.com/14647

From-SVN: r229096

gcc/go/gofrontend/MERGE

-a4bcd319d98ddc52b3e7d16ec87d92aad868ab05
+302d8abbc499e28088d758ae8b2c024d8e50b9b3
 
 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

@@ -5601,7 +5601,9 @@ Binary_expression::do_check_types(Gogo*)
       if (left_type->integer_type() == NULL)
 	this->report_error(_("shift of non-integer operand"));
 
-      if (!right_type->is_abstract()
+      if (right_type->is_string_type())
+        this->report_error(_("shift count not unsigned integer"));
+      else if (!right_type->is_abstract()
 	  && (right_type->integer_type() == NULL
 	      || !right_type->integer_type()->is_unsigned()))
 	this->report_error(_("shift count not unsigned integer"));