Fix fold of 0-result 0-trip-count `affine.for`

Folding 0-result ops is tricky. An 0-trip-count `affine.for` that does
not return any results is not erased at the end of its usual fold
method. Attempting fold it to only results in an infinite loop of folds
on the same op. Fix this by attempting for fold away only those
`affine.for` ops that return a non-zero number of results.

Reviewed By: bondhugula

Differential Revision: https://reviews.llvm.org/D151546