hdl,back,sim: accept .as_signed() and .as_unsigned() on LHS.

These operators are ignored when they are encountered on LHS, as
the signedness of the assignment target does not matter in Amaranth.
.as_signed() appears on LHS of assigns to signed aggregate fields.

amaranth/sim/_pyrtl.py

@@ -279,6 +279,8 @@ class _LHSValueCompiler(_ValueCompiler):
         return gen
 
     def on_Operator(self, value):
+        if value.operator in ("u", "s"):
+            return self(value.operands[0])
         raise TypeError # :nocov:
 
     def on_Slice(self, value):