The table.drop bytecode "drops" the backing storage for an element
initializer. In the V8 implementation, this is a nop, other than
updating a per-instance boolean array so that two drops of the same
segment or a drop of an active segment will trap.

This is implemented with inline code in TurboFan in order to be symmetric
to memory.drop, but could as easily be a runtime call to be supported in
Liftoff.

R=mstarzinger@chromium.org
CC=​binji@chromium.org
BUG=v8:7747

Change-Id: Ic017398eaa764dd3a9ff19523453ff7142c9abf6
Reviewed-on: https://chromium-review.googlesource.com/c/1408996Reviewed-by: Ben Smith <binji@chromium.org>
Commit-Queue: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58817}