cmd/compile: refactor GOSSAHASH debugging to make it usable outside ssa package.

cmd/compile: refactor GOSSAHASH debugging to make it usable outside ssa package.

I've needed this more than once in the past, I hack it in,
then throw it away, seems sensible to make the change and
save it.

Fixes #53937.

Change-Id: I7fe886b1c93d73cbf553bed587f2c30f0f5d5a0b
Reviewed-on: https://go-review.googlesource.com/c/go/+/418015
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>