Convert String array index/length hash to BitField

src/x64/macro-assembler-x64.h

Index: src/x64/macro-assembler-x64.h
diff --git a/src/x64/macro-assembler-x64.h b/src/x64/macro-assembler-x64.h
index 34d034ac03172a7f05bf3ba8560792d369eccf3a..ae5a95983af36afa8d7f4744452167f8e41f72be 100644
--- a/src/x64/macro-assembler-x64.h
+++ b/src/x64/macro-assembler-x64.h
@@ -1014,10 +1014,18 @@ class MacroAssembler: public Assembler {
   void DecodeField(Register reg) {
     static const int shift = Field::kShift;
     static const int mask = Field::kMask >> Field::kShift;
-    shrp(reg, Immediate(shift));
+    if (shift != 0) {
+      shrp(reg, Immediate(shift));
+    }
     andp(reg, Immediate(mask));
   }
 
+  template<typename Field>
+  void DecodeFieldToSmi(Register reg) {
+    andp(reg, Immediate(Field::kMask));
+    shlp(reg, Immediate(kSmiShift - Field::kShift));
+  }
+
   // Abort execution if argument is not a number, enabled via --debug-code.
   void AssertNumber(Register object);