[inliner] additional bits and pieces of documentation and assertions

--HG--
extra : amend_source : 64c8bc497c1e4060d38d806d655bd7e5486c1b9c
[inliner] return result versus parameter mutation, former deemed more readable

--HG--
extra : amend_source : ffebe06ba1295fd496e604313ea1bd4ead45dcb2
[inliner] documentation
[inliner] break method up, to enable delaying specializeGraphToArguments()
[inliner] both parts of what used to be a single method now invoked in sequence
[inliner] readability
Backed out changeset: a750e0d83535 (cache last receiver stamp in MethodCallTargetNode)
add ComputeLoopFrequenciesPhase after canonicalizer
[inliner] trickle up, thus making more visible, graph copying
[inliner] removed a method, lost nothing (but code is more readable afterwards)
[inliner] no need to alias a final field
Merge