pan/bi: Add bi_count_succ_reads helper

pan/bi: Add bi_count_succ_reads helper

The number of register writes in a tuple must be bounded by a number
based on the number of register reads in the succeeding tuple, since
writes and reads are interleaved. This helper calculates that number of
reads, noting that not every read actually counts - if the preceding
tuple writes to a read source, that will become a temporary instead of
consuming a register slot.

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/8354>