This seems quite useful for the development, so you don't need to wait
all the kernels to be re-compiled when working on a new feature, which
speeds up re-iteration.
Marked as an advanced option, so if it doesn't work so well in practice
it's safe to revert anyway.
This way util_simd.cpp would not require modifications
if/when SSE2 is suddenly supported on 32bit platforms.
This also allowed to unleash some issues with util_simd.h
related on the fact that there size_t and int are actually
the same types.
This makes the code a bit easier to understand, and might come in handy
if we want to reuse more Embree code.
Differential Revision: https://developer.blender.org/D482
Code by Brecht, with fixes by Lockal, Sergey and myself.
