Silence shorten-64-to-32 compiler warning

BUG=aomedia:2123

Change-Id: I425a170297fea91e38b698be09ca25719f46821e
diff --git a/aom_dsp/simd/v128_intrinsics_arm.h b/aom_dsp/simd/v128_intrinsics_arm.h
index d4fec42..c96a080 100644
--- a/aom_dsp/simd/v128_intrinsics_arm.h
+++ b/aom_dsp/simd/v128_intrinsics_arm.h
@@ -375,7 +375,8 @@
   uint64x2_t m = vpaddlq_u32(vpaddlq_u16(vpaddlq_u8(
       vandq_u8(vreinterpretq_u8_s64(a),
                vreinterpretq_u8_u64(vdupq_n_u64(0x8040201008040201ULL))))));
-  return v64_u64(v64_ziplo_8(v128_high_v64((v128)m), v128_low_v64((v128)m)));
+  return v64_low_u32(
+      v64_ziplo_8(v128_high_v64((v128)m), v128_low_v64((v128)m)));
 #endif
 }