Revert "Reland "[compiler] Move the main pipeline's code assembly pass into the background.""

This reverts commit 7b080310.

Reason for revert: There's still another issue.

Original change's description:
> Reland "[compiler] Move the main pipeline's code assembly pass into the background."
> 
> This is a reland of 66b54ab1
> Original change's description:
> > [compiler] Move the main pipeline's code assembly pass into the background.
> > 
> > R=bmeurer@chromium.org
> > 
> > Bug: v8:6048
> > Change-Id: I60bc35c02b5460416c3b0e2872fc72ebf9b808a5
> > Reviewed-on: https://chromium-review.googlesource.com/563386
> > Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
> > Commit-Queue: Georg Neis <neis@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#46499}
> 
> TBR=bmeurer@chromium.org
> 
> Bug: v8:6048
> Change-Id: Ic841abc893c96271dc4dec7a0d19ba6a8b39164e
> Reviewed-on: https://chromium-review.googlesource.com/565140
> Reviewed-by: Georg Neis <neis@chromium.org>
> Commit-Queue: Georg Neis <neis@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#46563}

TBR=neis@chromium.org

Change-Id: I07ac9d44324d7cfed72531c7084bc8ba9ddef799
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:6048
Reviewed-on: https://chromium-review.googlesource.com/567059Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46565}

src/compiler/pipeline.cc

@@ -661,11 +661,11 @@ PipelineCompilationJob::Status PipelineCompilationJob::PrepareJobImpl() {
 
 PipelineCompilationJob::Status PipelineCompilationJob::ExecuteJobImpl() {
   if (!pipeline_.OptimizeGraph(linkage_)) return FAILED;
-  pipeline_.AssembleCode(linkage_);
   return SUCCEEDED;
 }
 
 PipelineCompilationJob::Status PipelineCompilationJob::FinalizeJobImpl() {
+  pipeline_.AssembleCode(linkage_);
   Handle<Code> code = pipeline_.FinalizeCode();
   if (code.is_null()) {
     if (info()->bailout_reason() == kNoReason) {