pan/mdg: Use analysis to set .cont/.last flags

pan/mdg: Use analysis to set .cont/.last flags

Corresponds roughly to what we analyze. Note that "terminate AND
execute" is a contradiction (rather: it's equivalent to just
terminating), hence why there are only three possibilities for the
states of the flags:

   .cont = continue, don't execute
   .last = don't continue, don't execute
   .cont.last = continue and execute

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5014>