pan/bi: Add bi_foreach_instr_in_clause iterators

pan/bi: Add bi_foreach_instr_in_clause iterators

These are convenient for post-sched passes, in particular register
allocation. They work by noting the underlying linked list of
instructions in the block must be preserved by scheduling. It isn't
necessary iterate the clause structure directly, it can simply be used
to bound a iteration within the block by recalling clauses are strictly
contained in a single block.

   <alyssa> I don't think I'm enough of a C ninja to write fancy iterator macros yet.
   <zmike> sometimes those can get pretty mind-bendy

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8723>