Fix merge of sgml-syntax-propertize-rules

Fix merge of sgml-syntax-propertize-rules

During the merge of emacs-26, the sgml-syntax-propertize-rules part of
2019-01-17 "* lisp/textmodes/sgml-mode.el: Try and fix bug#33887." got
lost in the conflict against 2019-05-09 "Recognize single quote
attribute values in nxml and sgml (Bug#35381)".
* lisp/textmodes/sgml-mode.el (sgml-syntax-propertize-rules): Reapply
the 2019-01-17 change to speed up sgml-syntax-propertize-rules, taking
into account the 2019-05-09 which means we have to handle single
quotes as well.