cmd/internal/obj/x86: minor clean-up in span6

cmd/internal/obj/x86: minor clean-up in span6

* Reduce the scope of q.
* Remove duplicate handling of AADJSP.
* Move ab declaration closer to use.
* Collapse nested if statements.
* Change declaration of n for increased readability in context.
* Simplify AADJSP handling.

Passes toolstash-check.

Change-Id: I046369477db567f2f7c4a9c8d400ec9dd9c32f3f
Reviewed-on: https://go-review.googlesource.com/c/go/+/168342
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>