Fix time_reversed_for_flows handling feedback incorrectly

[Strilanc]

import stim
c = stim.Circuit("""
    R 1
    M 1
    CX rec[-1] 0
""")
p = stim.PauliString("Z")
f = stim.Flow(input=p, output=p)
assert c.has_flow(f)
c_inv, (f_inv,) = c.time_reversed_for_flows([f])
assert c_inv.has_flow(f_inv)

Currently raises an exception instead of succeeding.