[SCEV] Add stricter verification option.

Currently -verify-scev only fails if there is a constant difference
between two BE counts. This misses a lot of cases.

This patch adds a -verify-scev-strict options, which fails for any
non-zero differences, if used together with -verify-scev.

With the stricter checking, some unit tests fail because
of mis-matches, especially around IndVarSimplify.

If there is no reason I am missing for just checking constant deltas, I
am planning on looking into the various failures.

Reviewers: efriedma, sanjoy.google, reames, atrick

Reviewed By: sanjoy.google

Differential Revision: https://reviews.llvm.org/D68592

llvm-svn: 374535