We can't consistently rewire the successor blocks of an unreachable node to
disconnect them from the graph when we are trying to maintain the schedule.
Instead simply leave the code there. As a future optimization we could add a
proper scheduled dead code elimination phase which can deal with this.

As a side-effect, one of the tests sees a int64 DeadValue, so add support for that
in the instruction selector.

BUG=chromium:1083272,chromium:1083763,chromium:1084953,v8:9684

Change-Id: I69a6feaeef4eae62110392e27ea848b28bccf787
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2209061
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Auto-Submit: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67953}