add comments and delete dead code

R=ken
OCL=22078
CL=22080
diff --git a/src/cmd/gc/go.h b/src/cmd/gc/go.h
index ce8dd42..5499b71 100644
--- a/src/cmd/gc/go.h
+++ b/src/cmd/gc/go.h
@@ -313,6 +313,8 @@
 	OLITERAL, OREGISTER, OINDREG,
 	OCONV, OCOMP, OKEY,
 	OBAD,
+	
+	OEXTEND,	// 6g internal
 
 	OEND,
 };