minor improvements on dce #3502

Job	Run time
macOS / Release / homebrew-clang	8m 21s
Ubuntu / Release / gcc	21m 38s
Ubuntu / Debug / gcc	6m 10s
Ubuntu / Release / clang	18m 10s
macOS / Debug / homebrew-clang	3m 32s
Ubuntu / Debug / clang	6m 33s
macOS / Release / system-clang	8m 11s
Windows / Release / cl	23m 9s
macOS / Debug / system-clang	2m 25s
Windows / Release / clang	32m 23s
Windows / Release / clang-cl	26m 27s
Windows / Debug / cl	18m 4s
Windows / Debug / clang	18m 13s
Windows / Debug / clang-cl	10m 18s
	3h 23m 34s

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