Fix the hole loading optimization.
- Holes are only ever loaded as double or tagged.
- Change to tagged has to deoptimize on undefined (no implicit
  conversions from double the hole NaN -> tagged undefined).

BUG=
R=jkummerow@chromium.org

Review URL: https://chromiumcodereview.appspot.com/16099006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14829 ce2b1a6d-e550-0410-aec6-3dcde31c8c00