[Polly] Don't crash on invalid delinearization result.

In certain cases, it's possible for delinearization to decide one of the
array dimensions should be some function of an induction variable inside
the scop.  Make sure if this happens, we refuse to use those dimensions
for delinearization.

Usually, we end up rejecting the scop before it actually crashes, but it
looks like it's possible to slip past other checks in certain cases
involving smax expressions.

Fixes a crash that started showing up this week on the polly AOSP
builder.  As far as I can tell, this is a longstanding issue, though;
it was just exposed by better SCEV analysis of smin expressions.

Differential Revision: https://reviews.llvm.org/D61807

llvm-svn: 360708