JIT: Support consuming profile guided optimization data for optimization

[redknightlois]

Currently the native generation is able to use PGO and as stated by @jkotas on https://github.com/dotnet/coreclr/issues/6759#issuecomment-240809700 it is going to be supported at CoreCLR eventually (already supported at the desktop framework). Being able to allow the JIT to consume that information without the need to generate a native executable would allow software developers to pack that data into their executables to get the same benefits in a portable way.

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[pgavlin]

cc @dotnet/jit-contrib

[DemiMarie]

Even better: support generating profiling information at run-time, and then consuming the same information. This is what Java does, but would require being able to re-JIT code and discard old versions.

[mattwarren]

This is what Java does, but would require being able to re-JIT code and discard old versions.

The CLR currently has the ability to do this, see ReJIT: A How-To Guide, although AFAIK it's only currently utilised by the Profiling API, not by the run-time itself.

[AndyAyersMS]

We have this ability now.