runtime: use fast path when chan is closed and empty

[delichik]

Check channel emptiness in recv regardless of blocking mode.
If the channel is empty and closed, recheck emptiness and
return immediately without acquiring the lock.

This improves receive performance on closed channels. The
only cost is one additional empty() check for normal
blocking receives, which is negligible compared to lock
acquisition.

Updates #77522

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Patch Set 1: Code-Review+1 Commit-Queue+1

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