[DAGCombiner] Extend ISD::ABDS/U combine to handle more cases.

The current ABD combine doesn't quite work for SVE because only a
single scalable vector per scalar integer type is legal (e.g. for
i32, <vscale x 4 x i32> is the only legal scalable vector type).

This patch extends the combine to also trigger for the cases when
operand extension must be retained.

Differential Revision: https://reviews.llvm.org/D115739