Convert String array index/length hash to BitField

src/x64/macro-assembler-x64.cc

Index: src/x64/macro-assembler-x64.cc
diff --git a/src/x64/macro-assembler-x64.cc b/src/x64/macro-assembler-x64.cc
index acff18596712c4cd746c2ed6a0bab772c0fbf0f9..521b10f9f94a6b34b0384f0840d53195c6700097 100644
--- a/src/x64/macro-assembler-x64.cc
+++ b/src/x64/macro-assembler-x64.cc
@@ -548,16 +548,10 @@ void MacroAssembler::IndexFromHash(Register hash, Register index) {
   // reserved for it does not conflict.
   ASSERT(TenToThe(String::kMaxCachedArrayIndexLength) <
          (1 << String::kArrayIndexValueBits));
-  // We want the smi-tagged index in key. Even if we subsequently go to
-  // the slow case, converting the key to a smi is always valid.
-  // key: string key
-  // hash: key's hash field, including its array index value.
-  andp(hash, Immediate(String::kArrayIndexValueMask));
-  shrp(hash, Immediate(String::kHashShift));
-  // Here we actually clobber the key which will be used if calling into
-  // runtime later. However as the new key is the numeric value of a string key
-  // there is no difference in using either key.
-  Integer32ToSmi(index, hash);
+  if (!hash.is(index)) {
+    movl(index, hash);
+  }
+  DecodeFieldToSmi<String::ArrayIndexValueBits>(index);
 }