hdl._nir: AssignmentList: fix comb_edges_to #3561

Job	Run time
check-links	46s
document	53s
smoke (amaranth-lang/amaranth-boards)	40s
smoke (amaranth-lang/amaranth-stdio)	38s
test (3.8)	2m 19s
smoke (amaranth-lang/amaranth-soc)	36s
test (3.9)	2m 26s
test (3.10)	2m 6s
test (3.11)	2m 31s
test (3.12)	3m 23s
test (pypy-3.8)	5m 9s
test (pypy-3.9)	4m 50s
test (pypy-3.10)	4m 53s
test (3.13-dev)	3m 11s
publish-docs	6s
publish-docs-dev	0s
required	1s
publish-package	40s
publish-schemas	8s
publish-release	0s
	35m 16s

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