hdl.ir: lower domains before resolving hierarchy conflicts.

Otherwise, two subfragments with the same local clock domain would
not be able to drive its clock or reset signals. This can be easily
hit if using two ResetSynchronizers in one module.

Fixes #265.

nmigen/hdl/ir.py

@@ -540,8 +540,8 @@ class Fragment:
 
         fragment = SampleLowerer()(self)
         new_domains = fragment._propagate_domains(missing_domain)
-        fragment._resolve_hierarchy_conflicts()
         fragment = DomainLowerer()(fragment)
+        fragment._resolve_hierarchy_conflicts()
         if ports is None:
             fragment._propagate_ports(ports=(), all_undef_as_ports=True)
         else: