Avoid array bounds check when allowed by guarded field.

runtime/vm/intermediate_language_x64.cc

Index: runtime/vm/intermediate_language_x64.cc
diff --git a/runtime/vm/intermediate_language_x64.cc b/runtime/vm/intermediate_language_x64.cc
index c5a1bf3af2a6a1a2ff56a8a6b044f5122cf1ba19..5ebc8f873ce24cf662da44e58a0643be96f52a1a 100644
--- a/runtime/vm/intermediate_language_x64.cc
+++ b/runtime/vm/intermediate_language_x64.cc
@@ -4437,6 +4437,24 @@ void CheckArrayBoundInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
     return;
   }
 
+  intptr_t guarded_array_length = -1;
+  if (array_ != NULL) {
+    if (array_->guarded_list_length() >= 0) {
+      guarded_array_length = array_->guarded_list_length();
+    }
+  }
+
+  if (index_loc.IsConstant() && guarded_array_length >= 0) {
+    const Object& index = Smi::Cast(index_loc.constant());
+    int64_t i = reinterpret_cast<int64_t>(index.raw());
+    if (i >= guarded_array_length) {
+      // We know this bounds check will trigger a deoptimization.
+      __ jmp(deopt);
+    }
+    // Nothing to emit.
+    return;
+  }
+
   if (index_loc.IsConstant()) {
     Register length = length_loc.reg();
     const Smi& index = Smi::Cast(index_loc.constant());