[mlir][gpu] Add support for unsigned integer extend in vector to gpu.subgroup_mma lowering

Unsigned integer types are supported in subgroup mma ops by matching
against arith.extui ops. This allows for subgroup_mma_compute ops with
mixed signedness which requires later conversions to handle this. SPIR-V
cooperative matrix ops support this while the lowering to WMMA does not.

Differential Revision: https://reviews.llvm.org/D143922