Rather than manually tracking basic blocks in the bytecode array builder,
use the existing dead code elimination to generate an implicit return iff
the block ending the bytecode is not dead by the time all statements have
been visited.

Change-Id: I9520486a523ec4e01bc203e9a847eb1f57b130b6
Reviewed-on: https://chromium-review.googlesource.com/c/1494756Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59951}