[SCEV] Delay strengthening of nowrap flags

Strengthening nowrap flags is relatively expensive. Make sure we
only do it if we're actually going to use the flags -- we don't
use them for many recursive invocations. Additionally, if we're
reusing an existing SCEV node, there's no point in trying to
strengthen the flags if we don't have any new baseline facts.

This change falls slightly short of being NFC, because the way
flags during add+addrec / mul+addrec folding are handled may be
more precise (as less operands are included in the calculation).