Suppress canonicalization of Unbox() instruction that can deoptimize.

runtime/vm/intermediate_language.cc

Index: runtime/vm/intermediate_language.cc
diff --git a/runtime/vm/intermediate_language.cc b/runtime/vm/intermediate_language.cc
index 59f0cb232b255b1a368a5c80c48d04b7e4fe768c..cdd9d7f7a85493b6f293ff03acbc281d79788488 100644
--- a/runtime/vm/intermediate_language.cc
+++ b/runtime/vm/intermediate_language.cc
@@ -789,6 +789,18 @@ bool Instruction::IsDominatedBy(Instruction* dom) {
 }
 
 
+bool Instruction::HasUnmatchedInputRepresentations() const {
+  for (intptr_t i = 0; i < InputCount(); i++) {
+    Definition* input = InputAt(i)->definition();
+    if (RequiredInputRepresentation(i) != input->representation()) {
+      return true;
+    }
+  }
+
+  return false;
+}
+
+
 void Definition::ReplaceWith(Definition* other,
                              ForwardInstructionIterator* iterator) {
   // Record other's input uses.
@@ -1994,7 +2006,7 @@ Definition* BoxInt64Instr::Canonicalize(FlowGraph* flow_graph) {
 
 
 Definition* UnboxInstr::Canonicalize(FlowGraph* flow_graph) {
-  if (!HasUses()) return NULL;
+  if (!HasUses() && !CanDeoptimize()) return NULL;
 
   // Fold away Unbox<rep>(Box<rep>(v)).
   BoxInstr* box_defn = value()->definition()->AsBox();
@@ -2250,19 +2262,12 @@ Instruction* BranchInstr::Canonicalize(FlowGraph* flow_graph) {
       return this;
     }
     ComparisonInstr* comp = replacement->AsComparison();
-    if ((comp == NULL) || comp->CanDeoptimize()) {
+    if ((comp == NULL) ||
+        comp->CanDeoptimize() ||
+        comp->HasUnmatchedInputRepresentations()) {
       return this;
     }
 
-    // Assert that the comparison is not serving as a pending deoptimization
-    // target for conversions.
-    for (intptr_t i = 0; i < comp->InputCount(); i++) {
-      if (comp->RequiredInputRepresentation(i) !=
-          comp->InputAt(i)->definition()->representation()) {
-        return this;
-      }
-    }
-
     // Replace the comparison if the replacement is used at this branch,
     // and has exactly one use.
     Value* use = comp->input_use_list();