Adds InterpreterCompilationJob as a sub-class of
CompilationJob, to enable off-thread bytecode
generation. Currently only used in
Interpreter::MakeBytecode.

As part of this change, CompilationJob is modified
to make it less specific to optimized compilation,
renaming the phases as follows:
 - CreateGraph -> PrepareJob
 - OptimizeGraph -> ExecuteJob
 - GenerateCode -> FinalizeJob

RegisterWeakObjectsInOptimizedCode is also moved out
of CompilationJob and instead becomes a static function
on Compiler.

BUG=v8:5203

Review-Url: https://codereview.chromium.org/2240463002
Cr-Commit-Position: refs/heads/master@{#38662}