[DAG] Move lshr narrowing from visitANDLike to SimplifyDemandedBits

Inspired by some of the cases from D145468

Let SimplifyDemandedBits handle the narrowing of lshr to half-width if we don't require the upper bits, the narrowed shift is profitable and the zext/trunc are free.

A future patch will propose the equivalent shl narrowing combine.

Differential Revision: https://reviews.llvm.org/D146121