The use of com_node() introduces a lot of extra stack frames, enough
to cause a stack overflow compiling test.test_parser with the standard
interpreter recursionlimit.  The com_node() is a convenience function
that hides the dispatch details, but comes at a very high cost.  It is
more efficient to dispatch directly in the callers.  In these cases,
use lookup_node() and call the dispatched node directly.

Also handle yield_stmt in a way that will work with Python 2.1
(suggested by Shane Hathaway)