Only convert power-of-two floor-division with non-negative denominator

floord(a,b) === a ashr log_2 (b) holds for positive and negative a's, but
shifting only makes sense for positive values of b. The previous patch did
not consider this as isl currently always produces postive b's. To avoid future
surprises, we check that b is positive and only then apply the optimization.

We also now correctly check the return value of the dyn-cast.

No additional test case, as isl currently does not produce negative
denominators.

Reported-by: David Majnemer <david.majnemer@gmail.com>
llvm-svn: 238927