runtime: clear isSending bit earlier

I've done some more testing of the new isSending field.
I'm not able to get more than 2 bits set. That said,
with this change it's significantly less likely to have even
2 bits set. The idea here is to clear the bit before possibly
locking the channel we are sending the value on, thus avoiding
some delay and some serialization.

For #69312

Change-Id: I8b5f167f162bbcbcbf7ea47305967f349b62b0f4
Reviewed-on: https://go-review.googlesource.com/c/go/+/617497
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Commit-Queue: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

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