i965: Add cases for ir_triop_vector_insert that assert.

i965: Add cases for ir_triop_vector_insert that assert.

brw_link_shader() unconditionally calls lower_vector_insert() with true
as the second parameter.  This means that both constant and variable
indexed expressions will get lowered, so we should never see this in the
backend.

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