[SCEV] Factor out part of wrap flag detection logic [NFC](try 2)

This is a cut down version of 1ec6e1 which was reverted due to a compile time issue.  The key changes made from that patch: 1) only infer the flags needed along each path, 2) be careful to preserve order of checks, and 3) avoid computing NW flags at all since we need to prove the stronger property (does not cross 0) in the caller anyways.

Assuming this doesn't trip regressions, I'm going to try weakening (1).  My end objective is to move flag inference into addrec construction.  If I can't weaken (1) without compile time impact, I'll have a problem.