Further refactor the pipeline to even run the first scheduler (part of
the effect control linearization) concurrently. This temporarily
disables most of the write barrier elimination, but we will get back to
that later.

Drive-by-fix: Remove the dead code from ChangeLowering, and stack
allocate the Typer in the pipeline. Also migrate the AllocateStub to a
native code builtin, so that we have the code object + a handle to it
available all the time.

CQ_INCLUDE_TRYBOTS=tryserver.v8:v8_linux64_tsan_rel
R=mstarzinger@chromium.org
BUG=v8:4969
LOG=n

Review-Url: https://codereview.chromium.org/1926023002
Cr-Commit-Position: refs/heads/master@{#35918}