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>
diff --git a/src/cmd/7g/ggen.go b/src/cmd/7g/ggen.go
index 86772d7..da5215a 100644
--- a/src/cmd/7g/ggen.go
+++ b/src/cmd/7g/ggen.go
@@ -230,7 +230,7 @@
 		// TODO(minux): add gins3?
 		p1.Reg = p1.To.Reg
 
-		p1.To.Reg = tm.Val.U.Reg
+		p1.To.Reg = tm.Reg
 		gins(optoas(gc.OMUL, t), &tr, &tm)
 		gc.Regfree(&tr)
 		gins(optoas(gc.OSUB, t), &tm, &tl)