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

runtime/vm/assembler_x64.h

Index: runtime/vm/assembler_x64.h
diff --git a/runtime/vm/assembler_x64.h b/runtime/vm/assembler_x64.h
index c865814f3a1c3c37e8ca31c0beb68f39d23251f5..f4ce1322db3769e491a87268ec71fee92904328e 100644
--- a/runtime/vm/assembler_x64.h
+++ b/runtime/vm/assembler_x64.h
@@ -861,6 +861,11 @@ class Assembler : public ValueObject {
     sarq(reg, Immediate(kSmiTagSize));
   }
 
+  void BranchIfNotSmi(Register reg, Label* label) {
+    testq(reg, Immediate(kSmiTagMask));
+    j(NOT_ZERO, label);
+  }
+
   void Align(int alignment, intptr_t offset);
   void Bind(Label* label);
   void Jump(Label* label) { jmp(label); }