Restructure JSObject::SetElement for performance.
Restructure JSObject::SetElement for performance.

Wins back ~1500 points on Octane/Gameboy that we lost with
https://codereview.chromium.org/11365111 (CL 12900), presumably
by lowering register pressure and/or handlification overhead.
Hopefully benefits other regressions as well.

R=verwaest@chromium.org
BUG=

Review URL: https://codereview.chromium.org/11275283

git-svn-id: b158db1e4b4ab85d4c9e510fdef4b1e8c614b15b@12949 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh