[AArch64] Add an all-in-one histogram intrinsic

Based on discussion from
https://discourse.llvm.org/t/rfc-vectorization-support-for-histogram-count-operations/74788

Current interface is:

llvm.experimental.histogram(<vecty> ptrs, <intty> inc_amount, <vecty> mask)

The integer type used by 'inc_amount' needs to match the type of the buckets in memory.

The intrinsic covers the following operations:
  * Gather load
  * histogram on the elements of 'ptrs'
  * multiply the histogram results by 'inc_amount'
  * add the result of the multiply to the values loaded by the gather
  * scatter store the results of the add

Supports lowering to histcnt instructions for AArch64 targets, and scalarization for all others at present.