[CostModel][AArch64] Improve the cost estimate of CTPOP intrinsic

Added a case for CTPOP to AArch64TTIImpl::getIntrinsicInstrCost so that
the cost estimate matches the codegen in
test/CodeGen/AArch64/arm64-vpopcnt.ll

Differential Revision: https://reviews.llvm.org/D103952