pan/bi: Set I->nr_dests, I->nr_srcs

pan/bi: Set I->nr_dests, I->nr_srcs

The builder is the primary producer of instructions, and generally the shape of
an instruction is fixed at build-time. It must set nr_dests/nr_srcs
appropriately. Likewise, when we modify sources later, we need to update
nr_srcs/nr_dests to keep everything consistent (and keep the tests passing).

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17794>