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 <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

josharian/go,nawawi/go,golang/go,CAFxX/go,josharian/go,cloudflare/go,golang/go,CAFxX/go,CAFxX/go,CAFxX/go,josharian/go,golang/go,akutz/go,josharian/go,CAFxX/go,josharian/go,akutz/go,akutz/go,cloudflare/go,akutz/go,akutz/go,josharian/go,cloudflare/go,cloudflare/go,nawawi/go,nawawi/go,cloudflare/go,josharian/go,CAFxX/go,josharian/go,cloudflare/go,golang/go,nawawi/go,CAFxX/go,akutz/go,golang/go,nawawi/go,akutz/go,cloudflare/go,nawawi/go,cloudflare/go,golang/go,akutz/go,CAFxX/go,golang/go,golang/go,nawawi/go,nawawi/go