JIT: Share suspension epilogs in async transformation

[jakobbotsch]

If we have more than one await, then create a shared GT_RETURN_SUSPEND block and use that for suspension always.

Pretty modest R2R size diffs for x64.

R2R size diffs x64

Assembly                                               R2R base     R2R diff  R2R Delta    R2R %
--------                                            -----------     --------  ---------    -----
System.IO.Compression.dll                               458,752      454,656    -4.0 KB   -0.89%
System.Linq.AsyncEnumerable.dll                       1,220,608    1,216,512    -4.0 KB   -0.34%
System.Net.Http.dll                                   1,622,016    1,617,920    -4.0 KB   -0.25%
System.Private.Xml.dll                                7,548,928    7,544,832    -4.0 KB   -0.05%

Detailed asp.net+runtimeasync crossgen2 diffs:
https://gist.github.com/jakobbotsch/ab57d97f2ea63f30ada41cb57713c779

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[Copilot]

Pull request overview

This PR changes the CoreCLR JIT async transformation to share a single GT_RETURN_SUSPEND “shared return” block when there are multiple awaits, so multiple suspension points can branch to one epilog.

Changes:

• Refactors await discovery into FindAwaits(...), tracking normal vs tail-awaits and total await counts.
• Transforms tail awaits in a first pass (TransformTailAwaits(...)) and re-scans when needed.
• Introduces CreateSharedReturnBB() to create a shared GT_RETURN_SUSPEND block when #awaits > 1.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/coreclr/jit/async.h	Adds declarations for await discovery, tail-await batch transform, and shared return block creation.
src/coreclr/jit/async.cpp	Implements the refactor and shared return block creation, and updates tail-await suspension to use m_sharedReturnBB.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

[jakobbotsch]

/azp run runtime-coreclr superpmi-diffs

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.