RegExpObject::defineOwnProperty() does not need to compare values if no descriptor...

RegExpObject::defineOwnProperty() does not need to compare values if no descriptor value is specified.
https://bugs.webkit.org/show_bug.cgi?id=177600
<rdar://problem/34710985>

Reviewed by Saam Barati.

JSTests:

* stress/regress-177600.js: Added.

Source/JavaScriptCore:

According to http://www.ecma-international.org/ecma-262/8.0/#sec-validateandapplypropertydescriptor,
section 9.1.6.3-7.a.ii, we should only check if the value is the same if the
descriptor value is present.

* runtime/RegExpObject.cpp:
(JSC::RegExpObject::defineOwnProperty):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@223645 268f45cc-cd09-0410-ab3c-d52691b4dbfc