Reviewed by Geoffrey Garen.

        Make RegExpCache a weak map
        https://bugs.webkit.org/show_bug.cgi?id=61554

        Switch to a weak map for the regexp cache, and hide that
        behaviour behind RegExp::create.

        When a RegExp is compiled it attempts to add itself to
        the "strong" cache.  This cache is a simple round-robin
        buffer as was the old strong cache.  Happily this can
        be smaller than the old strong cache as RegExps are only
        added when they're compiled so it is under less pressure
        to evict.

        * bytecompiler/NodesCodegen.cpp:
        (JSC::RegExpNode::emitBytecode):
        * runtime/RegExp.cpp:
        (JSC::RegExp::RegExp):
        (JSC::RegExp::create):
        (JSC::RegExp::match):
        * runtime/RegExp.h:
        (JSC::RegExp::gcShouldInvalidateCode):
        (JSC::RegExp::hasCode):
        (JSC::RegExp::key):
        * runtime/RegExpCache.cpp:
        (JSC::RegExpCache::lookupOrCreate):
        (JSC::RegExpCache::RegExpCache):
        (JSC::RegExpCache::isReachableFromOpaqueRoots):
        (JSC::RegExpCache::finalize):
        * runtime/RegExpCache.h:
        * runtime/RegExpConstructor.cpp:
        (JSC::constructRegExp):
        * runtime/RegExpPrototype.cpp:
        (JSC::regExpProtoFuncCompile):
        * runtime/StringPrototype.cpp:
        (JSC::stringProtoFuncMatch):
        (JSC::stringProtoFuncSearch):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@87445 268f45cc-cd09-0410-ab3c-d52691b4dbfc