infinite loop in NIR builder on a trivial comb loop

[wanda-phi]

The following results in an infinite loop:

from amaranth import *
import amaranth.back.rtlil

m = Module()
a = Signal()
m.d.comb += a.eq(a)

amaranth.back.rtlil.convert(m, ports=[])

[whitequark]

Related to #704.