freedreno/ir3: do better job of marking convergence points

Fixes:
dEQP-GLES3.functional.shaders.switch.switch_in_do_while_loop_dynamic_vertex
dEQP-GLES3.functional.shaders.switch.switch_in_do_while_loop_dynamic_fragment

Signed-off-by: Rob Clark <robdclark@chromium.org>
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