This introduces unreachable state into load elimination. We mark state
as unreachable if we know statically that a map check would fail.
When processing effect phis, we disconnect unreachable state's
control from the effect phi's merge, and point it to RuntimeAbort.
The control input to the merge is then updated with Dead. Dead
code elimination prunes the merge, phis and effect phis.

Bug: v8:6396
Change-Id: I01874b576e548747a915c7b645b96ebaa6f6700d
Reviewed-on: https://chromium-review.googlesource.com/730754
Commit-Queue: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48810}