[llvm] Add an all-in-one histogram intrinsic, along with lowering for AArch64 (PR #88106)

[Graham Hunter via llvm-commits]

huntergr-arm wrote:

Rebased, addressed comments. I've added scalarization of the intrinsic for targets which don't have an appropriate vector instruction to map this to.

https://github.com/llvm/llvm-project/pull/88106