i965/vec4: Add a brw->gen >= 6 assertion in three-source emitters.

Three source instructions didn't exist until Gen6.  vec4_generator has
assertions to catch this, but catching it in the visitor provides a
nicer backtrace.

Cc: "10.1" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Acked-by: Eric Anholt <eric@anholt.net>
(cherry picked from commit ffde483f3c)

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

@@ -128,6 +128,7 @@ vec4_visitor::emit(enum opcode opcode)
    vec4_instruction *							\
    vec4_visitor::op(dst_reg dst, src_reg src0, src_reg src1, src_reg src2)\
    {									\
+      assert(brw->gen >= 6);						\
       return new(mem_ctx) vec4_instruction(this, BRW_OPCODE_##op, dst,	\
 					   src0, src1, src2);		\
    }