This adds assertions to generator support functions that distinguish
between old-style and new-style generators which make sure only one
of those styles is actually used. Even though normal functions can soon
be mixed (bytecode and baseline code at the same time), generator
functions are still exclusively in only one tier.

R=neis@chromium.org
BUG=v8:5265

Review-Url: https://codereview.chromium.org/2233863002
Cr-Commit-Position: refs/heads/master@{#38541}