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 <mknyszek@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>