Squeeze the layout of expression nodes a bit.

Again 112MB less peak memory usage in the bug mentioned below. :-)
Routed all writes to to_boolean_types_ through its setter on the way.

BUG=417697
LOG=y
R=mvstanton@chromium.org

Review URL: https://codereview.chromium.org/615423006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24393 ce2b1a6d-e550-0410-aec6-3dcde31c8c00