[mlir] Add map_nested_foreach_thread_to_gpu_blocks op to transform dialect

This revision adds a new op `map_nested_foreach_thread_to_gpu_blocks` to transform dialect.
If `generate_gpu_launch` argument is given, the op first generates `gpu_launch`. Otherwise, `target` must be `gpu_launch`. The op searches top level `scf.foreach_threads` inside the `gpu_launch` and distributes them with gpu.block_id attribute.
Loop mapping is explicit and given by the map_nested_foreach_thread_to_gpu_blocks op. Mapping is done one-to-one, therefore the loops disappear.
It also adds `gpu dialect` as dependent since the new op can create `gpu::LaunchOp` for given `scf::ForeachThreadOp`.

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D134190