Immutability: Missing write barrier in optimized bytecode

[xFrednet]

Bug found by Vanja:

class C:
    def __init__(self):
        self.a = [1,2,3]

    def f(self):
        self.a.append(4)
        freeze(self.a)

foo = C()       #     foo.a: [1, 2, 3]
foo.f()         # ok, foo.a: [1, 2, 3, 4]
foo.f()         # ok, foo.a: [1, 2, 3, 4, 4]
foo.a.append(5) # TypeError: object of type list is immutable

This looks similar to other bugs I saw with regions. It's very likely that C.f gets optimized and the optimized bytecode is missing the write barrier