Refactor vectorization patterns

This CL removes the reliance of the vectorize pass on the specification of a `fastestVaryingDim` parameter. This parameter is a restriction meant to more easily target a particular loop/memref combination for vectorization and is mainly used for testing.

This also had the side-effect of restricting vectorization patterns to only the ones in which all memrefs were contiguous along the same loop dimension. This simple restriction prevented matmul to vectorize in 2-D.

this CL removes the restriction and adds the matmul test which vectorizes in 2-D along the parallel loops. Support for reduction loops is left for future work.

PiperOrigin-RevId: 240993827