Fix DCE/Subst01 to work under lambdas #357

Job	Run time
docker-master	1h 41m 57s
test-standalone (master, dev, default)	58s
publish-standalone-dry-run	14s
build-wasm-of-ocaml (master, 4.14.1)	13m 15s
generated-files (master)	23m 25s
build-js-of-ocaml (master, 4.11.1)	18m 0s
standalone-haskell (master)	53m 3s
test-amd64	2h 18m 12s
validate-docker-master	2m 14s
deploy-js-wasm-of-ocaml	1m 4s
docker-check-all	2s
	5h 52m 24s

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