VM: Array bounds checks that don't deoptimize for precompiled code.

runtime/vm/assembler_arm64.h

Index: runtime/vm/assembler_arm64.h
diff --git a/runtime/vm/assembler_arm64.h b/runtime/vm/assembler_arm64.h
index 9085bf01cda7acb73a37223f1eecb70b388739bc..c39e8e3261060ba40c545d02f31193024424f98f 100644
--- a/runtime/vm/assembler_arm64.h
+++ b/runtime/vm/assembler_arm64.h
@@ -1219,6 +1219,11 @@ class Assembler : public ValueObject {
     LslImmediate(dst, src, kSmiTagSize);
   }
 
+  void BranchIfNotSmi(Register reg, Label* label) {
+    tsti(reg, Immediate(kSmiTagMask));
+    b(label, NE);
+  }
+
   void Branch(const StubEntry& stub_entry,
               Register pp,
               Patchability patchable = kNotPatchable);