When canonicalizing UnboxInt32([tr], BoxInt64()) make sure to mark resulting converter truncating.

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 a4d70faf9b5a513af6b9a315108e3234a4cefd57..956797c49bf934dbaad44a68e0871532cf068a4b 100644
--- a/runtime/vm/intermediate_language.cc
+++ b/runtime/vm/intermediate_language.cc
@@ -1208,6 +1208,8 @@ bool UnboxInt32Instr::CanDeoptimize() const {
     return !is_truncating() &&
         !RangeUtils::Fits(value()->definition()->range(),
                           RangeBoundary::kRangeBoundaryInt32);
+  } else if (is_truncating() && value()->definition()->IsBoxInteger()) {
+    return false;
   } else if ((kSmiBits < 32) && value()->Type()->IsInt()) {
     // Note: we don't support truncation of Bigint values.
     return !RangeUtils::Fits(value()->definition()->range(),
@@ -2063,7 +2065,7 @@ Definition* UnboxIntegerInstr::Canonicalize(FlowGraph* flow_graph) {
           box_defn->value()->CopyWithType(),
           (representation() == kUnboxedInt32) ?
               GetDeoptId() : Isolate::kNoDeoptId);
-      if ((representation() == kUnboxedInt32) && !CanDeoptimize()) {
+      if ((representation() == kUnboxedInt32) && is_truncating()) {
         converter->mark_truncating();
       }
       flow_graph->InsertBefore(this, converter, env(), FlowGraph::kValue);