Updates a number of AST operations to avoid dereferencing handles
such that they can safely be called off-thread. Also adds a
HandleDereferenceMode argument to some operations where handles are
compared. If handle dereferencing is allowed, the handles are compared
directly, if not then their locations are compared (which relies on the
handles being created in a CanonicalHandleScope).

BUG=v8:5203
TBR=adamk@chromium.org

Review-Url: https://codereview.chromium.org/2223523002
Cr-Commit-Position: refs/heads/master@{#38526}