[mlir][spirv] Add cooperative matrix load op

Implement cooperative matrix load for the `SPV_KHR_cooperative_matrix`
extension: https://github.com/KhronosGroup/SPIRV-Registry/blob/master/extensions/KHR/SPV_KHR_cooperative_matrix.html.

Also some minor fixes in common code for custom parsing.

Reviewed By: antiagainst

Differential Revision: https://reviews.llvm.org/D155616