[IndVarSimplify] use the "canonical" way to infer no-wrap.

Summary:
rL225282 introduced an ad-hoc way to promote some additions to nuw or
nsw.  Since then SCEV has become smarter in directly proving no-wrap;
and using the canonical "ext(A op B) == ext(A) op ext(B)" method of
proving no-wrap is just as powerful now.  Rip out the existing
complexity in favor of getting SCEV to do all the heaving lifting
internally.

This change does not add any unit tests because it is supposed to be a
non-functional change.  Tests added in rL225282 and rL226075 are valid
tests for this change.

Reviewers: atrick, majnemer

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D7981

llvm-svn: 231306