Reland and fix bug: Don't store redundant compile-type at values.

runtime/vm/intermediate_language.cc

Index: runtime/vm/intermediate_language.cc
===================================================================
--- runtime/vm/intermediate_language.cc	(revision 41847)
+++ runtime/vm/intermediate_language.cc	(working copy)
@@ -2051,8 +2051,15 @@
       if ((representation() == kUnboxedInt32) && !CanDeoptimize()) {
         converter->mark_truncating();
       }
-      flow_graph->InsertBefore(this, converter, env(), FlowGraph::kValue);
-      return converter;
+      // Only replace if the replacement has CanDeoptimize is consistent with
+      // the original instructions. We may have removed environments already.
+      // Due to the way type propagation works currently, the CompileType of the
+      // converter input may not be the same (more general) than the information
+      // about the input of this unboxing instructions.
+      if (CanDeoptimize() == converter->CanDeoptimize()) {
+        flow_graph->InsertBefore(this, converter, env(), FlowGraph::kValue);
+        return converter;
+      }
     }
   }