[DAG] computeKnownBits - add ISD::MULHS/MULHU/SMUL_LOHI/UMUL_LOHI handling

Reuse the existing KnownBits multiplication code to handle the 'extend + multiply + extract high bits' pattern for multiply-high ops.

Noticed while looking at the codegen for D88785 / D98587 - the patch helps division-by-constant expansion code in particular, which suggests that we might have some further KnownBits div/rem cases we could handle - but this was far easier to implement.

Differential Revision: https://reviews.llvm.org/D98857