[SCEV] Be more aggressive around proving no-wrap

... for AddRec's in loops for which SCEV is unable to compute a max
tripcount.  This is not a problem for "normal" loops[0] that don't have
guards or assumes, but helps in cases where we have guards or assumes in
the loop that can be used to constrain incoming values over the backedge.

This partially fixes PR27691 (we still don't handle the NUW case).

[0]: for "normal" loops, in the cases where we'd be able to prove
no-wrap via isKnownPredicate, we'd also be able to compute a max
tripcount.

llvm-svn: 269211