i965/vec4: Silence warnings about unhandled interpolation ops

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

src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp

@@ -1768,6 +1768,9 @@ vec4_visitor::visit(ir_expression *ir)
    case ir_unop_unpack_half_2x16_split_x:
    case ir_unop_unpack_half_2x16_split_y:
    case ir_binop_pack_half_2x16_split:
+   case ir_unop_interpolate_at_centroid:
+   case ir_binop_interpolate_at_sample:
+   case ir_binop_interpolate_at_offset:
       unreachable("not reached: should not occur in vertex shader");
    case ir_binop_ldexp:
       unreachable("not reached: should be handled by ldexp_to_arith()");