[mlir][arith][NFC] Simplify narrowing patterns with a wrapper type

Add a new wraper type that represents either of `ExtSIOp` or `ExtUIOp`.
This is to simplify the code by using a single type, so that we do not
have to use templates or branching to handle both extension kinds.

Reviewed By: antiagainst

Differential Revision: https://reviews.llvm.org/D149485