[ARM] Single source VMOVNT

Our current lowering of VMOVNT goes via a shuffle vector of the form
<0, N, 2, N+2, 4, N+4, ..>. That can of course also be a single input
shuffle of the form <0, 0, 2, 2, 4, 4, ..>, where we use a VMOVNT to
insert a vector into the top lanes of itself. This adds lowering of that
case, re-using the existing isVMOVNMask.

Differential Revision: https://reviews.llvm.org/D96065