Properly annotate control flow convergence points

From https://gitlab.freedesktop.org/mesa/mesa/blob/master/src/freedreno/ir3/ir3_legalize.c#L456

/* we want to mark points where divergent flow control re-converges
 * with (jp) flags.  For now, since we don't do any optimization for
 * things that start out as a 'do {} while()', re-convergence points
 * will always be a branch or jump target.  Note that this is overly
 * conservative, since unconditional jump targets are not convergence
 * points, we are just assuming that the other path to reach the jump
 * target was divergent.  If we were clever enough to optimize the
 * jump at end of a loop back to a conditional branch into a single
 * conditional branch, ie. like:

We now do optimizations that make the fall through of a branch (exiting a do-while loop) a convergence point and it breaks

dEQP-GLES3.functional.shaders.switch.switch_in_do_while_loop_dynamic_vertex

and a similar case in the GLES2 list.