cmd/internal/obj: clean up checkaddr
cmd/internal/obj: clean up checkaddr

Coalesce identical cases.
Give it a proper doc comment.
Fix comment locations.
Update/delete old comments.

Passes toolstash-check -all.

Change-Id: I88d9cf20e6e04b0c1c6583e92cd96335831f183f
Reviewed-on: https://go-review.googlesource.com/38442
Run-TryBot: Josh Bleecher Snyder <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Dave Cheney <bfcdf3e6ca6cef45543bfbb57509c92aec9a39fb@cheney.net>
Reviewed-by: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>

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