[AArch64][GlobalISel] Legalize narrow type G_CTPOPs

Using `clampScalar` here because we ought to mark s128 as custom eventually.

(Right now, it will just fall back.)

With this legalization, we get the same code as SDAG:
https://godbolt.org/z/TneoPKrKG

Differential Revision: https://reviews.llvm.org/D100908