[MLIR][Affine] Fix generateUnrolledLoop utility

generateUnrolledLoop was assuming that the yielded value is always generated in
the Block corresponding to the loop being unrolled. Thus, it was updating the
last yielded values with it's cloned value. However, if the yielded value is not
generated in the same Block then the cloned value and it's corresponding mapping
won't exist, resulting in a crash. Fix this.

Reviewed By: bondhugula

Differential Revision: https://reviews.llvm.org/D146931