[mlir] Add map_nested_foreach_thread_to_gpu_threads op to transform dialect

This revision adds a new op `map_nested_foreach_thread_to_gpu_threads` to transform dialect. The op searches `scf.foreach_threads` inside the `gpu_launch` and distributes them with `gpu.thread_id` attribute.

Loop mapping is explicit and given by the `map_nested_foreach_thread_to_gpu_threads` op. Mapping is done one-to-one, therefore the loops dissappear.

The dynamic trip count or trip count that are larger than thread size are not supported for the time being. However, we can indeed support them by generating a loop inside with cyclic scheduling.

For the time being, trip counts that are dynamic or bigger than thread sizes are not supported. However, in the future the compiler can indeed generate a loop with static cyclic scheduling to support these cases.

Current mechanism allows `scf.foreach_threads` to be siblings or nested. There cannot be interleaving code between the loops when they are nested.

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D133950