hdl._nir: AssignmentList: fix comb_edges_to [0.5 backport] #3563

Job	Run time
document	47s
check-links	47s
smoke (amaranth-lang/amaranth-boards)	35s
test (3.8)	2m 40s
smoke (amaranth-lang/amaranth-stdio)	38s
test (3.9)	2m 8s
smoke (amaranth-lang/amaranth-soc)	44s
test (3.10)	2m 31s
test (3.11)	3m 37s
test (3.12)	3m 26s
test (pypy-3.8)	5m 25s
test (pypy-3.9)	5m 16s
test (pypy-3.10)	5m 7s
test (3.13-dev)	3m 21s
publish-docs	5s
publish-docs-dev	0s
required	1s
publish-schemas	8s
publish-package	42s
publish-release	0s
	37m 58s

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