This patch contains broken test cases for the
transform.gpu.map_forall_to_blocks op caused by two regressions
introduced in https://reviews.llvm.org/D155941: (1) it may be that
more dimensions are extracted from an existing scf.forall op than
there are dimensions to map to, in which case we crash in a call to
zip, which expects two iterations of the same length; and (2) if
generate_gpu_launch is set, the grid has to be 3D, but previously,
lower-dimensional grids were also accepted, and it would be great if
these inputs were extended with unit dimensions.
See also https://reviews.llvm.org/D156527, where I attempted a fix for
both problems.