[AArch64] Basic demand elements for some intrinsics

A lot of neon intrinsics work lane-wise, meaning that non-demanded
elements in and not demanded out. This teaches that to
AArch64TTIImpl::simplifyDemandedVectorEltsIntrinsic for some simple
single-input truncate intrinsics, which can help remove unnecessary
instructions.

Differential Revision: https://reviews.llvm.org/D117097