Some simd.h refactoring
Some simd.h refactoring

Use 'elements' enum member and loops, rather than explicit array
indexing, where possible, to make less of the non-SIMD bits hard-coded
to vector length.

lgritz/oiio,YangYangTL/oiio,OpenImageIO/oiio,micler/oiio,YangYangTL/oiio,micler/oiio,YangYangTL/oiio,lgritz/oiio,YangYangTL/oiio,lgritz/oiio,micler/oiio,bdeluca/oiio,micler/oiio,OpenImageIO/oiio,lgritz/oiio,OpenImageIO/oiio,bdeluca/oiio,bdeluca/oiio,bdeluca/oiio,OpenImageIO/oiio