nir/cf: Fix dominance metadata in the dead control flow pass.

nir/cf: Fix dominance metadata in the dead control flow pass.

The NIR control flow modification API churns the block structure,
splitting blocks, stitching them back together, and so on.  Preserving
information about block dominance is hard (and probably not worthwhile).

This patch makes nir_cf_extract() throw away all metadata, like we do
when adding/removing jumps.

We then make the dead control flow pass compute dominance information
right before it uses it.  This is necessary because earlier work by the
pass may have invalidated it.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>