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/racewalk.go b/src/cmd/internal/gc/racewalk.go
index 85225c3..4f7ed2d 100644
--- a/src/cmd/internal/gc/racewalk.go
+++ b/src/cmd/internal/gc/racewalk.go
@@ -492,7 +492,7 @@
 	if isartificial(b) {
 		return false
 	}
-	class := int(b.Class)
+	class := b.Class
 
 	// BUG: we _may_ want to instrument PAUTO sometimes
 	// e.g. if we've got a local variable/method receiver