[mlir][arith] Rename addui_carry to addui_extended

The goal is to make the naming of the future `_extended` ops more
consistent. With unsigned addition, the carry value/flag and overflow
bit are the same, but this is not true when it comes to signed addition.

Also rename the second result from `carry` to `overflow`.

Reviewed By: antiagainst

Differential Revision: https://reviews.llvm.org/D139569