[DAG] computeKnownBits - ISD::ABDS is zero in the high bits if the input has multiple sign bits

[RKSimon]

Split off from #94344

ABDS alive: https://alive2.llvm.org/ce/z/7_z2Vc

If the inputs are sign extended, then the absolute result is guaranteed to be zero in the (NumSignBits - 1) upper bits.

Noticed while working on #92576