rewrite_descriptor(): Fix bug that dropped the first TEXT node from

rewrite_descriptor():  Fix bug that dropped the first TEXT node from
                       <description> elements.  Add another sanity
                       check to make sure a special case only becomes
                       involved for <opcodedesc> and <opcodedescni>
                       elements.

fixup_paras_helper():  Clean up control flow.

Everywhere that checks node.nodeType==ELEMENT and node.tagName now
just check node.nodeName (nodeName is guaranteed for all node types
and will be special values that don't conflict with GIs for
non-ELEMENT nodes).  Simplifies a number of tests and improves
readability in a lot of places.