leading_zeros: emit one fewer add/sub instruction for leading_zeros #277

Job	Run time
Lint	6m 19s
Test (ubuntu-20.04, x86_64-unknown-linux-gnu)	15m 25s
Test (macOS-latest, x86_64-apple-darwin)	11m 54s
Test (windows-2022, x86_64-pc-windows-msvc)	29m 52s
Test (ubuntu-20.04, aarch64-linux-android, x86_64-unknown-linux-gnu)	8m 21s
cargo-deny	1m 7s
test_success	4s
	1h 13m 2s

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