i965: Add support for gl_VertexID and gl_InstanceID.

i965: Add support for gl_VertexID and gl_InstanceID.

The compiler setup for these VF-uploaded attributes looks a little
cheesy with mixing system values and real VBO-sourced attributes.  It
would be nice if we could just compute the ATTR[] map to GRF index up
front and use it at visit time instead of using ir->location in the
ATTR file.  However, we don't know the reg_offset at
visit(ir_variable *) time, so we can't do the mapping that early.

Fixes piglit vertexid test.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>