Is it safe to use a suction cup on a screen with a crack in the middle

yes its safe to use in that way, i heard sometimes people use broken or cracked screen mobile for long duration such as 1 year

TLDR answer: no.

If you have to remove the screen, plan on replacing it. Even an unbroken screen can easily be damaged when removing it from the S7, ask me how I know. But it is possible to remove without damage if you are careful. I imagine even a cracked screen such as you describe could be removed without destroying it, but even more care would have to be taken.

But, on the S7 very few part replacements require the screen to be removed; obviously the screen (glass, screen, digitizer assembly), home button/fingerprint sensor, and the two capactive buttons.

The back glass must be removed for nearly all repairs except the rear camera lens.