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

This reverts commit 1ec6e1eb.

This change causes a significant compile-time regression:
https://llvm-compile-time-tracker.com/compare.php?from=dd0b8b94d0796bd895cc998dd163b4fbebceb0b8&to=1ec6e1eb8a084bffae8a40236eb9925d8026dd07&stat=instructions

I assume that this is due to the non-NFC part of the change, which
now performs expensive nowrap inference even for nowrap flags that
are not used by the particular code.