fhdl: fix mismatch between _can_lower() and _lower_specials_step().

The conditions in them were different: _can_lower() checked for
presence of the lower() override method, and _lower_specials_step()
would only lower the special if lower() is present and returns
something other than None.

Remove _can_lower() and instead lower specials until no more can
be lowered. This is also both simpler and faster.

migen/fhdl/tools.py

@@ -346,21 +346,14 @@ def _lower_specials_step(overrides, specials):
     return f, lowered_specials
 
 
-def _can_lower(overrides, specials):
-    for special in specials:
-        cl = special.__class__
-        if cl in overrides:
-            cl = overrides[cl]
-        if hasattr(cl, "lower"):
-            return True
-    return False
-
-
 def lower_specials(overrides, f):
     lowered_specials = set()
-    while _can_lower(overrides, f.specials):
+    while True:
         fs, lowered_specials_step = _lower_specials_step(overrides, f.specials)
         f += fs
-        lowered_specials |= lowered_specials_step
-        f.specials -= lowered_specials_step
+        if lowered_specials_step:
+            lowered_specials |= lowered_specials_step
+            f.specials -= lowered_specials_step
+        else:
+            break
     return f, lowered_specials