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pan/va: Fix flaky BARRIERs

Alyssa Rosenzweig requested to merge alyssa/mesa:va/debug-flake into main 
For some unknown reason, waiting for general slots (at least for memory stores)
doesn't work properly on a BARRIER instruction. We need to wait for all general
slots right before issuing the BARRIER in addition to the general wait on the
BARRIER itself. I don't know if this is a hardware bug or some hideous
gate-saving quirk, but I observe the Mali-G78 DDK using the same workaround,
which implies this really is necessary.

Fixes rare flakes in:

   dEQP-GLES31.functional.compute.shared_var.work_group_size.float_128_1_1

Note that the flakes from that test are extremely timing dependent. Without this
change, that test is racy but we almost always win the race. Reproducing the
issue reliably requires high system load (e.g. running the CTS in the
background) and simultaneously running that test a large number of times.

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