in 6g -r (the rsc flag),
rewrite heap pointer moves as xchg.

R=ken
OCL=22665
CL=22665
diff --git a/src/cmd/6g/cgen.c b/src/cmd/6g/cgen.c
index 4d6e112..43497ad 100644
--- a/src/cmd/6g/cgen.c
+++ b/src/cmd/6g/cgen.c
@@ -7,6 +7,7 @@
 /*
  * generate:
  *	res = n;
+ * simplifies and calls gmove.
  */
 void
 cgen(Node *n, Node *res)