Speaking of which, I propose that codestr[] array is made one byte
longer and RETURN_VALUE opcode wrote in that extra byte.  It will be
removed by this patch anyway (if it is accepted), but we'll remove a way
to accidentally disable peephole optimizer.

I mean, it would cost almost nothing, yet will prevent making some
functions non-optimized.  E.g. like this:

def foo(x):
    if x >= 0:
        return x * 2
    raise ValueError