[SCEV][NFC] Sink initialization of SymbolicMaxNotTaken from ExitLimit constructor to its callers

Preserves current behavior (always select Exact if known, otherwise select Constant Max).
This is the final preparation step before letting each particular computation way to decide
how exactly it should be computed. Functional improvement is coming shortly as follow-up.

Differential Revision: https://reviews.llvm.org/D139402
Reviewed By: nikic, fhahn