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>
10 jobs for !1194 with wip/deqp-fixes in 11 minutes and 58 seconds (queued for 2 seconds)
latest detached
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  Containers Build
passed debian #412224

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  Build+Test
passed meson-clang #412226

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passed meson-clover #412230

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passed meson-main #412229

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passed meson-swr-glvnd #412225

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passed meson-vulkan #412231

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passed scons-llvm #412233

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passed scons-nollvm #412232

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passed scons-swr #412227

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passed scons-win64 #412228

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