tracer: return default name on unrecognized opcode.

The default name is more commonly returned on code such as:

    x, y = Signal(), Signal()

The case where the opcode is not recognized is only encountered
when older Amaranth is ran on a newer Python interpreter (with more
opcodes).

Returning None instead of a name here caused issues in the RTLIL
backend, which would incorrectly use $\d+ names for ports, since
the RTLIL backend assumed the name of a signal is always a string.

Fixes #733.

amaranth/tracer.py

@@ -24,8 +24,9 @@ def get_var_name(depth=2, default=_raise_exception):
             call_index += 2
         else:
             break
-    if call_opc not in ("CALL_FUNCTION", "CALL_FUNCTION_KW", "CALL_FUNCTION_EX", "CALL_METHOD", "CALL"):
-        return None
+    if call_opc not in ("CALL_FUNCTION", "CALL_FUNCTION_KW", "CALL_FUNCTION_EX",
+                        "CALL_METHOD", "CALL"):
+        return default
 
     index = call_index + 2
     while True: