[DAG] Attempt shl narrowing in SimplifyDemandedBits (REAPPLIED)

If a shl node leaves the upper half bits zero / undemanded, then see if we can profitably perform this with a half-width shl and a free trunc/zext.

Followup to D146121

Reapplied - moved after the ShrinkDemandedOp call; reuse the existing KnownBits result; ensure that we only attempt this if all the upper bits are demanded; 547dc461 should address the remaining regressions that were noticed in the previous commit.

Differential Revision: https://reviews.llvm.org/D155472