Crossgen2 - precompile generic instantiations (#2154)

Crossgen2 - precompile generic instantiations (#2154)

* Crossgen2 - precompile generic instantiations

This change adds precompilation of all methods on generic instantiations
of types that are encountered during compilation. In my powershell
startup time experiments, it shaved off about 110 methods from the ones
that needed to be jitted at runtime and looking at e.g.
System.Private.CoreLib.dll precompilation result, it is much closer to
the version we get from the old crossgen now.

* Create a new node type to carry the method dependecies