cmd/compile: in poset, improve panic strings and comments
cmd/compile: in poset, improve panic strings and comments

No functional changes.

Change-Id: I6f5e811e141dd09dc5c47ff2d37fae4c640315e3
Reviewed-on: https://go-review.googlesource.com/c/go/+/200862
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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