Weird. Ok, try opening the app drawer and using the search icon to search for the app..
You didn't happen to disable it at some point, did you? Go to Settings>Apps>Disabled, and see if it's listed there.
Forgive me, I don't have an M8. What about that Calculator Widget? Remember that widgets aren't displayed with all of the other apps in the App Drawer. They'll be listed separately with all of the other widgets.
Ok, here's another potentially stupid question for you--in that Settings>Apps>All screen, what happens when you scroll all the way down (without putting a search term in)? Do you see any disabled apps at the very bottom? I wonder if disabled apps don't show up if you use a search term like "cal" as you did.
Go into "Apps" and tap the 3 little dots at the top right of the screen and tap "hide/unhide apps". Scroll to "Calculator" and it should have a green box checked. Uncheck the box and save and your calculator app will now be visible in Apps.