HLoadRoot doesn't participate in representation inference, and its
represenation is not Tagged at code generation, which leads to incorrect
pointer map assignment and eventual stale pointer access after GC.

BUG=chromium:419036
LOG=Y
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/626383003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24410 ce2b1a6d-e550-0410-aec6-3dcde31c8c00