Fix representation of HLoadRoot.

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