[vm/compiler] Improve elimination of dead CatchBlockEntry Parameters

This change improves elimination of dead ParameterInstr instructions
by skiping over Unbox instructions when tracing for real uses.

Previously, ParameterInstr were removed if they we used only by other
dead Parameters or as inputs to dead phis. Now, Unbox instructions
are also allowed as long as they are also dead.

TEST=vm/cc/TryCatchOptimizer_DeadParameterElimination_Cyclic1
(with sound null safety)

Change-Id: Iba807ce43d10ff6e400abd614cc749d60f2454e1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/212900
Commit-Queue: Alexander Markov <alexmarkov@google.com>
Reviewed-by: Slava Egorov <vegorov@google.com>

runtime/vm/compiler/backend/redundancy_elimination.cc

@@ -3914,7 +3914,7 @@ class TryCatchAnalyzer : public ValueObject {
 
         for (auto phi : *join->phis()) {
           phi->mark_dead();
-          if (HasNonPhiUse(phi)) {
+          if (HasActualUse(phi)) {
             MarkLive(phi);
           }
         }
@@ -3922,7 +3922,7 @@ class TryCatchAnalyzer : public ValueObject {
     }
 
     for (auto info : parameter_info_) {
-      if (HasNonPhiUse(info->instr)) {
+      if (HasActualUse(info->instr)) {
         MarkLive(info->instr);
       }
     }
@@ -3962,6 +3962,8 @@ class TryCatchAnalyzer : public ValueObject {
           worklist_.Add(param);
         }
       }
+    } else if (UnboxInstr* unbox = defn->AsUnbox()) {
+      MarkLive(unbox->value()->definition());
     }
   }
 
@@ -4003,10 +4005,16 @@ class TryCatchAnalyzer : public ValueObject {
   }
 
   // Returns true if definition has a use in an instruction which is not a phi.
-  static bool HasNonPhiUse(Definition* defn) {
+  // Skip over Unbox instructions which may be inserted for unused phis.
+  static bool HasActualUse(Definition* defn) {
     for (Value* use = defn->input_use_list(); use != nullptr;
          use = use->next_use()) {
-      if (!use->instruction()->IsPhi()) {
+      Instruction* use_instruction = use->instruction();
+      if (UnboxInstr* unbox = use_instruction->AsUnbox()) {
+        if (HasActualUse(unbox)) {
+          return true;
+        }
+      } else if (!use_instruction->IsPhi()) {
         return true;
       }
     }