[mlir][ArmSME] Support decomposing constant splats into ArmSME tiles (#88762)

This adds a simple rewrite/legalization to decompose constant splats
larger than a single ArmSME tile into multiple SME virtual tile sized
splats. E.g. a constant splat to `vector<[8]x[8]xi32>` would decompose
into four `vector<[4]x[4]xi32>` splats.