md5: minor optimization in software backend (#755) #1374

Job	Run time
set-msrv / set-msrv	5s
minimal-versions / minimal-versions	43s
test (stable)	30s
test (1.85.0)	34s
build (stable, thumbv7em-none-eabi)	25s
build (stable, wasm32-unknown-unknown)	26s
build (1.85.0, thumbv7em-none-eabi)	26s
build (1.85.0, wasm32-unknown-unknown)	20s
	3m 29s

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