glsl: Exit when the shader IR contains an interface block instance

glsl: Exit when the shader IR contains an interface block instance

While writing the link_varyings::single_interface_input test, I
discovered that populate_consumer_input_sets assumes that all shader
interface blocks have been lowered to discrete variables.  Since there
is a pass that does this, it is a reasonable assumption.  It was,
however, non-obvious.  Make the code fail when it encounters such a
thing, and add a test to verify that behavior.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>