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 <cuong.manhle.vn@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/const.go

@@ -27,12 +27,12 @@ const (
 
 type Val struct {
 	// U contains one of:
-	// bool     bool when n.ValCtype() == CTBOOL
-	// *Mpint   int when n.ValCtype() == CTINT, rune when n.ValCtype() == CTRUNE
-	// *Mpflt   float when n.ValCtype() == CTFLT
-	// *Mpcplx  pair of floats when n.ValCtype() == CTCPLX
-	// string   string when n.ValCtype() == CTSTR
-	// *Nilval  when n.ValCtype() == CTNIL
+	// bool     bool when Ctype() == CTBOOL
+	// *Mpint   int when Ctype() == CTINT, rune when Ctype() == CTRUNE
+	// *Mpflt   float when Ctype() == CTFLT
+	// *Mpcplx  pair of floats when Ctype() == CTCPLX
+	// string   string when Ctype() == CTSTR
+	// *Nilval  when Ctype() == CTNIL
 	U interface{}
 }