[SCEV] Factor out part of wrap flag detection logic [NFC-ish]

In an effort to make code around flag determination more readable, and (possibly) prepare for a follow up change, factor out some of the flag detection logic.  In the process, reduce the number of locations we mutate wrap flags by a couple.

Note that this isn't NFC.  The old code tried for NSW xor (NUW || NW).  This is, two different paths computed different sets of wrap flags.  The new code will try for all three.  The result is that some expressions end up with a few extra flags set.