Do not eliminate GuardFieldInstr when guarding list length.

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 43dc0943c4faba4f2909f9ff7789f30af82d5458..355949f992e3a698b150147940a5154e3f13c51f 100644
--- a/runtime/vm/intermediate_language.cc
+++ b/runtime/vm/intermediate_language.cc
@@ -1579,6 +1579,11 @@ Instruction* GuardFieldInstr::Canonicalize(FlowGraph* flow_graph) {
     return NULL;  // Nothing to guard.
   }
 
+  if (field().guarded_list_length() != Field::kNoFixedLength) {
+    // We are still guarding the list length.
+    return this;
+  }
+
   if (field().is_nullable() && value()->Type()->IsNull()) {
     return NULL;
   }