This general mechanism ought to aid with structure watchpointing.
This is the non-buggy part of https://bugs.webkit.org/show_bug.cgi?id=112233. It's already been reviewed, and I will roll it in separately because it is generally more useful than the specific optimizations for by-string lookups.
Created attachment 193502 [details]
patch for landing
Landed in http://trac.webkit.org/changeset/146071