llvmpipe: Simplify and fix system variables fetch.

llvmpipe: Simplify and fix system variables fetch.

The system array values concept doesn't really because it expects the
system values to be fixed per call, which is wrong for gl_VertexID and
iffy for gl_SampleID.  So this patch does two things:

- kill the array, have emit_fetch_system_value directly pick the
  values it needs (only gl_InstanceID for now, as the previous code)

- correctly handle the expected type in emit_fetch_system_value

Signed-off-by: Olivier Galibert <galibert@pobox.com>
Reviewed-by: Brian Paul <brianp@vmware.com>