[NFCI][SCEV] Always refer to enum SCEVTypes as enum, not integer

The main tricky thing here is forward-declaring the enum:
we have to specify it's underlying data type.

In particular, this avoids the danger of switching over the SCEVTypes,
but actually switching over an integer, and not being notified
when some case is not handled.

I have updated most of such switches to be exaustive and not have
a default case, where it's pretty obvious to be the intent,
however not all of them.