i965: Add cases for ir_binop_vector_extract that assert.

i965: Add cases for ir_binop_vector_extract that assert.

do_vec_index_to_swizzle() should remove any vector extract operations
with a constant index.  It's unconditionally called from
do_common_optimization().

do_vec_index_to_cond_assign() should remove the rest, and it is
unconditionally called from brw_link_shader().  This means that we
should never see ir_binop_vector_extract in the backend.

Silences compiler warnings.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>