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: Ifcdd660dd69c6c4c1bc628961f4180a0b6ea4d9f
Reviewed-on: https://chromium-review.googlesource.com/567061Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46582}

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) {