Blender Git Loki

Cycles: Fix usage of AVX2 intrinsics in AVX kernel

While building the AVX kernel, util_avxf.h/avxb.h were using some AVX2 intrinsics,
these were never called, so it wasn't a run-time issue, but the intrinsics headers
on centos excluded the AVX2 prototypes when building the AVX kernel causing build errors.

This commit cleans up the improper usage of the AVX2 intrinsics and provides AVX
fallback implementations for future use.

Differential Revision: https://developer.blender.org/D3696