[compiler] Move the wasm pipeline's code assembly pass into the background.

R=ahaas@chromium.org, bmeurer@chromium.org

Bug: v8:6048
Change-Id: I5641cb1036fb0c6bf0803de3df9ddeb5787a93de
Reviewed-on: https://chromium-review.googlesource.com/563308Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46593}

src/compiler/pipeline.cc

@@ -807,6 +807,7 @@ PipelineWasmCompilationJob::ExecuteJobImpl() {
   }
 
   if (!pipeline_.ScheduleAndSelectInstructions(&linkage_, true)) return FAILED;
+  pipeline_.AssembleCode(&linkage_);
   return SUCCEEDED;
 }
 
@@ -816,7 +817,6 @@ size_t PipelineWasmCompilationJob::AllocatedMemory() const {
 
 PipelineWasmCompilationJob::Status
 PipelineWasmCompilationJob::FinalizeJobImpl() {
-  pipeline_.AssembleCode(&linkage_);
   pipeline_.FinalizeCode();
   return SUCCEEDED;
 }