Revert of [turbofan] Split CodeGenerator::GenerateCode into AssembleCode and FinishCodeObject. (patchset #3 id:40001 of https://codereview.chromium.org/2229243003/ )

Reason for revert:
There is a data race in the initialization of the Isolate::random_number_generator()

Original issue's description:
> [turbofan] Split CodeGenerator::GenerateCode into AssembleCode and FinishCodeObject.
>
> This CL splits CodeGenerator::GenerateCode into two new functions:
> AssembleCode and FinishCodeObject. AssembleCode does not access or
> modify the JS heap, which means that AssembleCode can be executed on
> background threads. FinishCodeObject allocates the generated code object
> on the JS heap and therefore has to be executed on the main thread.
>
> Implementation details:
> The GenerateCode function has been split just before out-of-line code is
> assembled. The reason is that code stubs may be generated when
> out-of-line code is assembled, which potentially allocates these code
> stubs on the heap.
>
> - Parts of initialization of the CodeGenerator has been moved from the
> constructor to an Initialize function so that we can instantiate an empty
> CodeGenerator object in PipelineData.
>
> R=bmeurer@chromium.org, mstarzinger@chromium.org, titzer@chromium.org
>
> Committed: https://crrev.com/03058a2187e32cc4080612181802086527c116a2
> Cr-Commit-Position: refs/heads/master@{#38604}

TBR=bmeurer@chromium.org,mstarzinger@chromium.org,titzer@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Review-Url: https://codereview.chromium.org/2240523003
Cr-Commit-Position: refs/heads/master@{#38605}