implement SSE::short_v division with _mm_div_ps
implement SSE::short_v division with _mm_div_ps

32-bit floats have enough precision for 16-bit integers. The overhead of
conversion between 1 16-bit integer vector and 2 32-bit float vectors is
small enough (at least on my Westmere CPU) to warrant this change.

VcDevel/Vc,VcDevel/Vc,VcDevel/Vc,chr-engwer/Vc,VcDevel/Vc,chr-engwer/Vc,chr-engwer/Vc