aco: don't always add logical edges from continue_break blocks to headers

aco: don't always add logical edges from continue_break blocks to headers

Otherwise, code like this will be broken:
loop {
   if (...) {
      break;
   } else {
      break;
   }
}
The continue_or_break block doesn't have any logical predecessors but it's
a logical predecessor of the header block. This liveness error breaks the
spiller in init_live_in_vars() (under "keep variables spilled on all
incoming paths") and eventually creates garbage reloads.

Fixes: 93c8ebfa ('aco: Initial commit of independent AMD compiler')
Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3257>