cmd/internal/gc: make class uint8 throughout

This prevents conversion noise.

No functional changes. Passes toolstash -cmp.

Change-Id: I238adf28680f875217827931a689ce7f19a9b371
Reviewed-on: https://go-review.googlesource.com/8164
Reviewed-by: David Chase <drchase@google.com>
diff --git a/src/cmd/internal/gc/walk.go b/src/cmd/internal/gc/walk.go
index 03e7926..c10201a 100644
--- a/src/cmd/internal/gc/walk.go
+++ b/src/cmd/internal/gc/walk.go
@@ -287,9 +287,9 @@
 			// so that reorder3 can fix up conflicts
 			var rl *NodeList
 
-			var cl int
+			var cl uint8
 			for ll := Curfn.Func.Dcl; ll != nil; ll = ll.Next {
-				cl = int(ll.N.Class) &^ PHEAP
+				cl = ll.N.Class &^ PHEAP
 				if cl == PAUTO {
 					break
 				}