This CLprepares the terrain for serialization/deserialization. It sets up
the instantiation stages such that we have a point wereh we can split off
obtaining the code from a snapshot, or snapshot. That point is after we
compile and produce the code table, but before we attach the
deoptimization info we use for stack tracing.

Opportunistically, performed more cleanup to improve maintainability:
- clarified sequential vs parallel compilation stages. FinishCompilation
was somewhat ambiguous in that it performed a few responsibilities:
compiling functions in the sequential case, and then populating the
linker and code tables.
- removed the "results" set, which is unnecessary. The linker simply
shares the function_code vector, and so do the compilation stages.
- populate the code table fixed array separately from compilation. This
falls out of the decisions above.

BUG=

Review-Url: https://codereview.chromium.org/2008043006
Cr-Commit-Position: refs/heads/master@{#36618}