[X86] Add basic vector handling for ISD::ABDS/ABDU (absolute difference) nodes

I'm intending to add generic legalization in the future, but for now I've added basic support to targets that have the necessary MIN/MAX support to expand to SUB(MAX(X,Y),MIN(X,Y)).

This exposed a couple of issues with the DAG combines - in particular we need to catch trunc(abs(sub(ext(x),ext(y)))) patterns earlier before the SSE/AVX vector trunc expansion folds trigger.

Differential Revision: https://reviews.llvm.org/D142288