[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 the easy FIXMEs.

I made the intrinsic form extensible, so that you specify the update operation instead of just assuming it's an addition. Have added to the LangRef.

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