The instruction sequence consumes a significant amount of memory, so it
should be deallocated as early as possible. This CL separates the zone
which is used by the code generator from the zone which is used by the
instruction selector. Thereby we can delete the instruction selector
zone, which contains the instruction sequence, already after
AssembleCode, and not only after FinalizeCode.

For WebAssembly this means that the instruction sequence gets deleted
on the background tasks already and does not stay alive until the
main threads deletes it.

R=bmeurer@chromium.org, neis@chromium.org
CC=mtrofin@chromium.org

Change-Id: I090a2140ca05ae3bcc66268b0eddb08846fea690
Reviewed-on: https://chromium-review.googlesource.com/566831
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Mircea Trofin <mtrofin@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46619}