[mlir][spirv] Add pattern to expand UMulExtended for WebGPU

This is needed because WGSL does not yet support extended multiplication
ops.

Set up pattern/pass stuff and handle the first op: `UMulExtended`.
`SMulExtended` handling will go to a separate patch.

Issue: https://github.com/llvm/llvm-project/issues/59563

Reviewed By: antiagainst

Differential Revision: https://reviews.llvm.org/D140995