cmd/compile: correct comments of gc.Val fields
cmd/compile: correct comments of gc.Val fields

Refer to Val.Ctype method instead of non-existed n.ValCtype one.

Change-Id: Ie0fbcae87d9f4f97632865676309d98e0b5138c4
Reviewed-on: https://go-review.googlesource.com/c/go/+/182561
Run-TryBot: Cuong Manh Le <54b4457f1dff6823379654eb5867596497ba782f@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>

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