This tracks instance type in load elimination, so that it can be used
to prune control flow with unreachable states.

(This is a cut down version of https://chromium-review.googlesource.com/c/v8/v8/+/727761),
the regression should be addressed by the map invalidation fix from
https://chromium-review.googlesource.com/c/v8/v8/+/730705.)

Bug: v8:6396
Change-Id: I3acab16ebbc0f1f16c7900a8d307deb84e1cb618
Reviewed-on: https://chromium-review.googlesource.com/732307
Commit-Queue: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48812}