[mlir][Arith][NFC] Migrate Arith dialect to the new fold API

This is the dialect in-tree with the most `fold` method implementations by far. This patch simply changes all implementations to make use of the new signature.

Admittedly, the code readability does not get a lot better in this case, simply due to most methods making use of `constFoldBinaryOp`. I did not modify that function or its interface as part of this patch, but might be something to consider in the future.

Differential Revision: https://reviews.llvm.org/D141490