r300: fix presub optimization bug, add deqp expectations for RV515.
This may not be a complete set of xfails, as I haven't been able to run dEQP-GLES2 to completion (GPU hangs at some point, no particular test seems to be guilty). But this will help me assess NIR-to-TGSI for the driver for !8044 (merged).
In the process of assessing NIR-to-TGSI, fix a bug that started causing more failures once NIR optimized more shaders.