Adding support for scalable vectors to the tiling and vectorization constructs in the Transform dialect

[banach-space]

[Implements #13334 ]

As per https://discourse.llvm.org/t/rfc-scalable-vectorisation-in-linalg/, add support for scalable tile sizes:

transform.structured.tile %0 [2, [4]]]

and scalable vector sizes:

transform.structured.masked_vectorize %0 vector_sizes [[4]]]

Ultimately, we should support the following (combined tiling and vectorisation):

%vscale_base = transform.param.const 4 : i64 -> index
%tile_size, %split_point = 
  transform.vector.compute_scalable_tile_size_and_split_point
  base(%vscale_base) (%op)
%main, %remainder = transform.structured.split %op after %split_point
%main_t = transform.structured.tile %main [%tile_size]
transform.structured.vectorize %main_t
transform.structured.masked_vectorize %remainder [[%vscale_base]]

CC @dcaballe

[banach-space]

The patches so far:

• https://reviews.llvm.org/D150944
• https://reviews.llvm.org/D150944
• https://reviews.llvm.org/D151892

This is sufficient for scalable vectorisation and tiling, but only allows for the trailing vector/tile size to be scalable.