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

[RKSimon]

Alive2: https://alive2.llvm.org/ce/z/a87fHU

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

ISD::ABDS should be similar as well.

Noticed while working on #92576

[RKSimon]

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