[mlir][gpu] Add support for integer types in gpu.subgroup_mma ops

The signedness is carried by `!gpu.mma_matrix` types to most closely
match the Cooperative Matrix specification which determines signedness
with the type (and sometimes the operation).

See: https://htmlpreview.github.io/?https://github.com/KhronosGroup/SPIRV-Registry/blob/master/extensions/NV/SPV_NV_cooperative_matrix.html

To handle the lowering from vector to gpu, ops such as arith.extsi are
pattern matched next to `vector.transfer_read` and `vector.contract` to
determine the signedness of the matrix type.

Enables s8 and u8 WMMA types in NVVM for the GPUToNVVM conversion.

Reviewed By: ThomasRaoux

Differential Revision: https://reviews.llvm.org/D143223