i965/vs: Add support for shadow comparitors with gather4

gather4_c's argument layout is straightforward -- refz just goes on the
end.

gather4_po_c's layout however -- the array index is replaced with refz.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
i965/fs: Add support for shadow comparitors with gather4

Note that gather4_po_c's parameters are too long for SIMD16. It might be
worth emitting 2xSIMD8 messages in this case at some point.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>