cmd/internal/gc, cmd/gc: move Reg from Val to Node

Val is used to hold constant values.
Reg was the odd duck out.

Generated using eg.

No functional changes. Passes toolstash -cmp.

Change-Id: Ic1de769a1f92bb02e09a4428d998b716f307e2f6
Reviewed-on: https://go-review.googlesource.com/8912
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/internal/gc/syntax.go

@@ -63,9 +63,12 @@ type Node struct {
 	// func
 	Func *Func
 
-	// OLITERAL/OREGISTER
+	// OLITERAL
 	Val Val
 
+	// OREGISTER, OINDREG
+	Reg int16
+
 	// ONAME
 	Ntype     *Node
 	Defn      *Node // ONAME: initializing assignment; OLABEL: labeled statement