i965: Fix swizzles for system values such as gl_InstanceID.

i965: Fix swizzles for system values such as gl_InstanceID.

visit(ir_variable *) sets dst_reg::writemask to the appropriate channel
for system values.  Unfortunately, visit(ir_dereference_variable *) then
calls swizzle_for_size, which for a float, sets the swizzle to .x.

This works for gl_VertexID, since we store it in the .x component (see
brw_draw_upload.c:732 - VID), but fails for gl_InstanceID (IID) since we
store it in the .y channel.

To fix this, avoid calling swizzle_for_size on ir_var_system_values.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>