[mlir][Transform] Add support for mapping to GPU warps and to linear ids

This revisions refactors the implementation of mapping to threads to additionally allow warps and linear ids to be specified.

`warp_dims` is currently specified along with `block_dims` as a transform attribute.

Linear ids on th other hand use the flattened block_dims to predicate on the first (linearized) k threads.
An additional GPULinearIdMappingAttr is added to the GPU dialect to allow specifying loops mapped to this new scheme.

Various implementation and transform op semantics cleanups are also applied.

Reviewed By: ThomasRaoux

Differential Revision: https://reviews.llvm.org/D146130