cmd/internal/obj/x86: clean up byteswapreg
cmd/internal/obj/x86: clean up byteswapreg

Make byteswapreg more Go like.

Change-Id: Ibdf3603cae9cad2b3465b4c224a28a4c4c745c2e
Reviewed-on: https://go-review.googlesource.com/38615
Run-TryBot: Dave Cheney <bfcdf3e6ca6cef45543bfbb57509c92aec9a39fb@cheney.net>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

ImJasonH/go,Cofyc/go,akutz/go,codestation/go,CAFxX/go,Cofyc/go,codestation/go,golang/go,cloudflare/go,derekmarcotte/go,linux-on-ibm-z/go,codestation/go,codestation/go,nawawi/go,CAFxX/go,linux-on-ibm-z/go,codestation/go,derekmarcotte/go,vsekhar/elastic-go,Cofyc/go,polyverse-security/go,josharian/go,vsekhar/elastic-go,akutz/go,Cofyc/go,josharian/go,linux-on-ibm-z/go,golang/go,nawawi/go,christopher-henderson/Go,ImJasonH/go,ImJasonH/go,nawawi/go,derekmarcotte/go,josharian/go,cloudflare/go,CAFxX/go,cloudflare/go,CAFxX/go,golang/go,codestation/go,linux-on-ibm-z/go,Cofyc/go,akutz/go,vsekhar/elastic-go,ImJasonH/go,golang/go,Cofyc/go,ImJasonH/go,nawawi/go,codestation/go,CAFxX/go,polyverse-security/go,derekmarcotte/go,derekmarcotte/go,cloudflare/go,vsekhar/elastic-go,codestation/go,golang/go,josharian/go,akutz/go,ImJasonH/go,golang/go,josharian/go,ImJasonH/go,CAFxX/go,CAFxX/go,akutz/go,cloudflare/go,Cofyc/go,josharian/go,vsekhar/elastic-go,polyverse-security/go,akutz/go,derekmarcotte/go,polyverse-security/go,derekmarcotte/go,linux-on-ibm-z/go,nawawi/go,polyverse-security/go,golang/go,golang/go,vsekhar/elastic-go,akutz/go,cloudflare/go,linux-on-ibm-z/go,linux-on-ibm-z/go,vsekhar/elastic-go,polyverse-security/go,Cofyc/go,akutz/go,vsekhar/elastic-go,ImJasonH/go,nawawi/go,CAFxX/go,josharian/go,derekmarcotte/go,nawawi/go,polyverse-security/go,cloudflare/go,polyverse-security/go,cloudflare/go,linux-on-ibm-z/go,josharian/go,nawawi/go