[Polly] [ScheduleOptimizer] Prevent incorrect tile size computation

Because Polly exposes parameters that directly influence tile size
calculations, one can setup situations like divide-by-zero.

Check against a possible divide-by-zero in getMacroKernelParams
and return early.

Also assert at the end of getMacroKernelParams that the block sizes
computed for matrices are positive (>= 1).

Tags: #polly

Differential Revision: https://reviews.llvm.org/D31708

llvm-svn: 299633