Start plumbing through support for the SPV_KHR_cooperative_matrix
extension: https://github.com/KhronosGroup/SPIRV-Registry/blob/master/extensions/KHR/SPV_KHR_cooperative_matrix.html.
Register the extension, add new coop matrix type, and add
spirv.KHR.CooperativeMatrixLength op to exercise it.
Make sure that mixing of the KHR and NV coop matrix extensions is not
allowed. Make cast verification more robust.
nit: attribute