migen/fhdl/tools: fix rename_clock_domain when new == old

Clock domain renaming should support new == old to allow programmatically determined clock domain renaming.

migen/fhdl/tools.py

@@ -256,6 +256,7 @@ def rename_clock_domain_expr(f, old, new):
 
 def rename_clock_domain(f, old, new):
     rename_clock_domain_expr(f, old, new)
+    if new != old:
         if old in f.sync:
             if new in f.sync:
                 f.sync[new].extend(f.sync[old])