Supporting simd16 vertex shaders involves packing the output of the
fetch shader appropriately, especially the vertexID buffers that have to
be formatted in one simd16 register, needed by the VS.

As part of this support, we needed to remove the 2nd JitManager, since it
was not accounting for vector width correctly.

USE_SIMD16_SHADERS is also split into two defines.  The additional
one (USE_SIMD16_VS) controls the width of the vertex shader (VS), while
the original one (USE_SIMD16_SHADERS) controls overall front end width.

Reviewed-by: Bruce Cherniak <bruce.cherniak@intel.com>