spirv: Lower BaseVertex to FIRST_VERTEX instead of BASE_VERTEX

spirv: Lower BaseVertex to FIRST_VERTEX instead of BASE_VERTEX

The base vertex in Vulkan is different from GL in that for non-indexed
primitives the value is taken from the firstVertex parameter instead
of being set to zero. This coincides with the new SYSTEM_VALUE_FIRST_VERTEX
instead of BASE_VERTEX.

v2 (idr): Add comment describing why SYSTEM_VALUE_FIRST_VERTEX is used
for SpvBuiltInBaseVertex.  Suggested by Jason.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com> [v1]
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>