[X86] Replace X86ISD::AVG with generic ISD::AVGCEILU

Pulled out of D106237, this replaces the X86ISD::AVG DAG node with the
generic ISD::AVGCEILU. It doesn't remove the detectAVGPattern method,
but the extra generic ISel matching does alter the existing test.

Differential Revision: https://reviews.llvm.org/D119073