[ARM] Teach DemandedVectorElts about VMOVN lanes

The class of instructions that write to narrow top/bottom lanes only
demand the even or odd elements of the input lanes. Which means that a
pair of VMOVNT; VMOVNB demands no lanes from the original input. This
teaches that to instcombine from the target hooks available through
ARMTTIImpl.

Differential Revision: https://reviews.llvm.org/D109325