i965: Fix partial variable access for geometry shaders in SSO mode.

i965: Fix partial variable access for geometry shaders in SSO mode.

Without varying packing, if a VS writes a compound variable, and the GS
only reads part of it, the base location of the variable may not
actually be in the VUE map.

To cope with this, we do lowering in terms of varying slots, add any
constant offsets to the base, and then do the VUE map remapping.  This
ensures we only look up VUE map entries for slots which actually exist.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>