ci: softpipe (and tgsi_exec cleanups)
This branch adds per-merge CI of the softpipe driver, based on the llvmpipe CI merged today. It adds another 8 minutes of CPU time (not wall time) to Mesa merges that don't directly touch softpipe (GLES2 and fractional GLES3 coverage), and as a compromise, 25 minutes on top of that for full GLES3 testing on merges that do directly touch softpipe.
I feel good about the 8m addition, but the 25 minute GLES3 run is a pretty big chunk of overhead. I was talking with @krh about maybe having a mesa/mesa-specific runner so that we can make sure that MRs don't end up blocked on CI jobs getting through fd.o shared runners (or a couple of Mesa devs pushing branches stealing the all the job slots from everyone else on fd.o).