[AArch64][GlobalISel] Implement custom legalization for s32 and s64 G_CTPOP

This is a partial port of AArch64TargetLowering::LowerCTPOP.

This custom lowering tries to uses NEON instructions to give a more efficient
CTPOP lowering when possible.

In the non-NEON/noimplicitfloat case, this should use the generic lowering
(see: https://godbolt.org/z/GcaPvWe4x). I think that's worth implementing after
implementing the widening code for s16/s8 though.

Differential Revision: https://reviews.llvm.org/D100399