Audit `string-match' usage in shr.el

Audit `string-match' usage in shr.el

* lisp/net/shr.el (shr-descend):
(shr-insert):
(shr-expand-url):
(shr-image-from-data):
(shr-dom-print):
(shr-parse-style):
(shr-tag-object):
(shr-tag-img):
(shr-render-td-1): Prefer string-match-p and string-search over
string-match where possible.