runtime: add predecessor method to treap
runtime: add predecessor method to treap

This change adds a method for computing a treap node's predecessor
to the treap, which will simplify the implementation of algorithms
used for heap growth scavenging.

For #14045.

Change-Id: Id203e4bd246db3504f2f0c5163ec36f4579167df
Reviewed-on: https://go-review.googlesource.com/c/144717
Run-TryBot: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>

nawawi/go,golang/go,akutz/go,akutz/go,josharian/go,cloudflare/go,golang/go,cloudflare/go,akutz/go,nawawi/go,josharian/go,cloudflare/go,josharian/go,nawawi/go,CAFxX/go,josharian/go,nawawi/go,cloudflare/go,cloudflare/go,CAFxX/go,josharian/go,CAFxX/go,akutz/go,CAFxX/go,akutz/go,cloudflare/go,golang/go,CAFxX/go,CAFxX/go,josharian/go,cloudflare/go,nawawi/go,golang/go,akutz/go,CAFxX/go,CAFxX/go,golang/go,josharian/go,golang/go,nawawi/go,golang/go,nawawi/go,cloudflare/go,akutz/go,josharian/go,golang/go,nawawi/go,akutz/go