[ARM][NEON] Improve vector popcnt lowering with PADDL (PR39281)

As I suggested on PR39281, this patch uses PADDL pairwise addition to widen from the vXi8 CTPOP result to the target vector type.

This is a blocker for moving more x86 code to generic vector CTPOP expansion (P32655 + D53258) - ARM's vXi64 CTPOP currently expands, which would generate a vXi64 MUL but ARM's custom lowering expands the general MUL case and vectors aren't well handled in LegalizeDAG - improving the CTPOP lowering was a lot easier than fixing the MUL lowering for this one case......

Differential Revision: https://reviews.llvm.org/D53257

llvm-svn: 344512