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.

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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.