cmd/compile: use more of internal/bisect in HashDebug

cmd/compile: use more of internal/bisect in HashDebug

Using more of internal/bisect gives us more that will be deleted
from base/hashdebug.go when we have updated the tools that
need the old protocol. It is also cheaper: there is no allocation to
make a decision about whether to enable, and no locking unless
printing is needed.

Change-Id: I43ec398461205a1a9e988512a134ed6b3a3b1587
Reviewed-on: https://go-review.googlesource.com/c/go/+/493736
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>