Blocks that contain soft deopts have no side effects.

src/hydrogen.cc

Index: src/hydrogen.cc
diff --git a/src/hydrogen.cc b/src/hydrogen.cc
index f2d7fe18a6afbeead8e3c97e8aac964e0ac4c14a..1f32447d4da18109209842815c805d134b0f278d 100644
--- a/src/hydrogen.cc
+++ b/src/hydrogen.cc
@@ -1414,6 +1414,11 @@ void HGlobalValueNumberer::ComputeBlockSideEffects() {
     int side_effects = 0;
     while (instr != NULL) {
       side_effects |= instr->ChangesFlags();
+      if (instr->IsSoftDeoptimize()) {
+        block_side_effects_[id] = 0;
+        side_effects = 0;
+        break;
+      }
       instr = instr->next();
     }
     block_side_effects_[id] |= side_effects;