turnip: remove lowering of indirect io
The first patch is from !5744 (merged)
My goal with this was to get turnip to compile saner tess shaders, but ended up fixing some other things too (turnip struct varyings, a random a3xx test).
There's failures in
arm64_a630_traces, although the result images look OK to me.. @anholt how am I supposed to get the reference images?