[MLIR][SPIRV] Add `UnsignedOp` trait.

This commit adds a new trait that can be attached to ops that have
unsigned semantics.

TODO:
- Check if other places in code can use the new attribute (possibly in this patch).
- Add a similar `SignedOp` attribute (in a new patch).

Reviewed By: antiagainst

Differential Revision: https://reviews.llvm.org/D94068