runtime: incorporate hbits advancement in scanobject into loop

This makes it clearer that i and hbits advance together.
As a bonus, it generates slightly better code.

Change-Id: I24d51102535c39f962a59c1a4a7c5c894339aa18
Reviewed-on: https://go-review.googlesource.com/c/go/+/309569


Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>