[ADT] Add APIntOps::abds signed absolute difference and rename absdiff -> abdu (#84791)

When I created APIntOps::absdiff, I totally missed that we already have ISD::ABDS/ABDU nodes, and we use this term in other places/targets as well.

I've added the APIntOps::abds implementation and renamed APIntOps::absdiff to APIntOps::abdu.

Given that APIntOps::absdiff is so young I don't think we need to create a deprecation wrapper, but I can if anyone thinks it important.

I'll do a KnownBits rename patch after this.