pan/bi: Use flow control lowering on Valhall

Logically at the same part of the compile pipeline as clause scheduling on
Bifrost. Lots of similarities, too. Now that we generate flow control only as a
late pass, various hacks in the compiler are no longer necessary and are
dropped.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <!16804>
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