Convert String array index/length hash to BitField

src/arm64/macro-assembler-arm64.cc

Index: src/arm64/macro-assembler-arm64.cc
diff --git a/src/arm64/macro-assembler-arm64.cc b/src/arm64/macro-assembler-arm64.cc
index cd7f0aeed4e5ea114ce9c20cb52de4f1fb1450d3..ff85931c01e8c59927155e24a028d83e43d256bf 100644
--- a/src/arm64/macro-assembler-arm64.cc
+++ b/src/arm64/macro-assembler-arm64.cc
@@ -3975,11 +3975,8 @@ void MacroAssembler::IndexFromHash(Register hash, Register index) {
   // conflict.
   ASSERT(TenToThe(String::kMaxCachedArrayIndexLength) <
          (1 << String::kArrayIndexValueBits));
-  // We want the smi-tagged index in key.  kArrayIndexValueMask has zeros in
-  // the low kHashShift bits.
-  STATIC_ASSERT(kSmiTag == 0);
-  Ubfx(hash, hash, String::kHashShift, String::kArrayIndexValueBits);
-  SmiTag(index, hash);
+  DecodeField<String::ArrayIndexValueBits>(index, hash);
+  SmiTag(index, index);
 }