Avoid duplicate ValueTask checks in transparent awaits

[jakobbotsch]

Classify ValueTask backing objects once in TransparentAwait and pass typed Task or IValueTaskSource instances to the suspend helpers. This mirrors the existing AsyncHelpers.Await implementation and removes repeated type checks and temporary ValueTask materialization from runtime-async tail awaits.

This is about 2% RPS improvement on the TechEmpower Arm64 JSON benchmarks. That benchmark was spending a not insignificant time in CastHelpers.IsInstanceOfClass invoked by the duplicated is Task<T> check that was inside AsyncHelpers.TransparentSuspend. The primary source of that comes from SocketReceiver.WaitForDataAsync.

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com

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[Copilot]

🔵 Needs a closer look

Pull request overview

This PR refactors the CoreCLR AsyncHelpers.TransparentAwait path for ValueTask/ValueTask<T> to classify the backing object once (Task vs IValueTaskSource*) and then call the appropriate suspend helper with the already-typed backing object, avoiding repeated runtime type checks during transparent/tail awaits.

Changes:

• Changes TransparentSuspend(ValueTask) / TransparentSuspend(ValueTask<T>) into TransparentSuspend(IValueTaskSource, short) / TransparentSuspend(IValueTaskSource<T>, short) and keeps Task-backed transparent suspension on the existing TransparentSuspend(Task) / TransparentSuspend(Task<T>) helpers.
• Updates TransparentAwait(ValueTask) / TransparentAwait(ValueTask<T>) to directly branch on task._obj and invoke TaskAwaiter.ValidateEnd or IValueTaskSource*.GetResult as appropriate without re-materializing/recursively re-checking the ValueTask.

File summaries

File	Description
src/coreclr/System.Private.CoreLib/src/System/Runtime/CompilerServices/AsyncHelpers.CoreCLR.cs	Moves ValueTask backing-object classification into TransparentAwait and passes typed backing objects to suspend helpers to reduce duplicated checks on the hot transparent-await path.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 0
• Review effort level: Lite

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