Allow merging of monomorphic accesses to tracked fields.

src/hydrogen-check-elimination.cc

Index: src/hydrogen-check-elimination.cc
diff --git a/src/hydrogen-check-elimination.cc b/src/hydrogen-check-elimination.cc
index 4fd769f9241e0186c5158067d82caa84ee1ccd4c..8eb760120d88cd958989c7cd9fccbbe65b77ed50 100644
--- a/src/hydrogen-check-elimination.cc
+++ b/src/hydrogen-check-elimination.cc
@@ -383,9 +383,9 @@ class HCheckTable : public ZoneObject {
   void ReduceLoadNamedField(HLoadNamedField* instr) {
     // Reduce a load of the map field when it is known to be a constant.
     if (!IsMapAccess(instr->access())) {
-      // Check if we introduce a map here.
-      if (!instr->map().IsNull()) {
-        Insert(instr, instr, instr->map());
+      // Check if we introduce field maps here.
+      if (instr->map_set().size() != 0) {
+        Insert(instr, instr, instr->map_set().Copy(phase_->zone()));
       }
       return;
     }