[SCEV] Introduce field for storing SymbolicMaxNotTaken. NFCI

ritht is initialized with either exact (if available) or
with constant max exit count. In the future, this can be improved.

Hypothetically this is not an NFC (it is possible that exact is not
known and max is known for a particular exit), but for how we use
it now it seems be an NFC (or at least I could not find an example
where it differs). constant max exit count. In the future, this can
be improved.

Differential Revision: https://reviews.llvm.org/D138699
Reviewed By: lebedev.ri