Function compress_portable 

fn compress_portable(value: u64, mask: u64) -> u64

compress without pext, by the number of kept bits k:

• k <= 2: the two lowest kept bits, tested directly
• k <= 16: one step per kept bit
• k >= 62: value with its at most two dropped bits removed
• otherwise compress_bytes

The two ends need no loop, so no loop exit to mispredict when k varies from word to word.