[RyuJit] Decouple igNum from strict layout order

[SingleAccretion]

The ultimate goal here is to remove the single-IG prolog constraint.

Currently, the igNum order matches the linked list order precisely (and thus the final layout order) precisely. It means we cannot generate any IGs "out of order". This change removes this restriction for a subset of IGs, those representing epilogs and funclet prologs, by hiding igNum behind and abstraction of "before [in layout order] / after [in layout order]" APIs. These APIs rely in the linked list to ascertain ordering between out-of-order groups.

Currently this is only exercised under a stress mode which splits all IGs into single-instruction groups. To this end, validation has been performed with this stress mode uncoditionally turned on for all methods [and all eligible IGs]:

1. All diffs have been validated to be expected.
2. Stress failures have also been investigated. The first one is a strange Helix issue with missing console logs, where the uploaded test file shows all tests having passed. The second is an interesting x86 GCStress timeout. I have run the failing test without the timeout and it passes (...after about an hour...), both before and after this change.

; The x86 timed-out test
23:57:11.640 Running test: async/returns/returns/returns.dll
App Exit Code: 259
Expected: 100
Actual: 259
END EXECUTION - FAILED
FAILED

Without stress, this is a zero-diff change since all our epilogs and funclet prologs fit into a single IG currently.

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[EgorBo]

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[SingleAccretion]

@dotnet/jit-contrib

[JulieLeeMSFT]

@AndyAyersMS , @kg , @adamperlin, PTAL.

[AndyAyersMS]

We lost diffs so will merge up to get new ones.

[kg]

LGTM but a second pair of eyes would be nice

[kg]

@dotnet/jit-contrib

[kg]

Thanks for your contribution!