Guard AVX2 implementation of psignbit in  PacketMath.h

Eigen/src/Core/arch/AVX/PacketMath.h

diff --git a/Eigen/src/Core/arch/AVX/PacketMath.h b/Eigen/src/Core/arch/AVX/PacketMath.h
index ebea1dc..2383e46 100644
--- a/Eigen/src/Core/arch/AVX/PacketMath.h
+++ b/Eigen/src/Core/arch/AVX/PacketMath.h
@@ -1837,7 +1837,11 @@
 }
 template <>
 EIGEN_STRONG_INLINE Packet8f psignbit(const Packet8f& a) {
+#ifdef EIGEN_VECTORIZE_AVX2
   return _mm256_castsi256_ps(_mm256_cmpgt_epi32(_mm256_setzero_si256(), _mm256_castps_si256(a)));
+#else
+  return _mm256_castsi256_ps(parithmetic_shift_right<31>(Packet8i(_mm256_castps_si256(a))));
+#endif
 }
 template <>
 EIGEN_STRONG_INLINE Packet8ui psignbit(const Packet8ui& a) {