i965/vec4: Redefine make_reg_for_system_value() to allow reuse in NIR->vec4 pass

i965/vec4: Redefine make_reg_for_system_value() to allow reuse in NIR->vec4 pass

The new virtual method is more flexible, it has a signature:

dst_reg *make_reg_for_system_value(int location, const glsl_type *type);

v2 (Jason Ekstrand):
   Use the new version in unit tests so make check passes again

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>