[CostModel][AArch64] Fix ctpop intrinsic cost when NEON is disabled.

If we don't have NEON, we use the generic fallback, which takes 12
instructions. Make sure the costs reflect that.

(On a related note, we could optimize the generic fallback a bit. It
currently uses sequences like lsr+and+add; if we use and+lsr+add
instead, we can fold the lsr into the add.)

Differential Revision: https://reviews.llvm.org/D133154