iris: Add maybe_flush calls to texture_barrier and memory_barrier

Otherwise, tests which loop on glMemoryBarrier may run us out of
batch space with piles of flushing.  (Ideally, we'd elide those bonus
PIPE_CONTROLs, but presumably this isn't that common of a case...)

Piglit's arb_pipeline_statistics_query-comp would hit this case after
some of the next patches remove other PIPE_CONTROLs with maybe_flushes.

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