Add ParticleIDWrapper::negative()

A cheaper way to check if a particle is valid.

Src/Particle/AMReX_Particle.H

@@ -110,9 +110,20 @@ struct ParticleIDWrapper
     AMREX_GPU_HOST_DEVICE AMREX_FORCE_INLINE
     void negate () noexcept
     {
-        // he leftmost bit is our id sign - we will XOR it with 1
+        // the leftmost bit is our id sign - we will XOR it with 1 to flip it
         m_idata ^= static_cast<uint64_t>(1) << 63;
     }
+
+    /** Check the sign of the id.
+     *
+     * Returns false if the id is positive, otherwise true (invalid particle).
+     */
+    AMREX_GPU_HOST_DEVICE AMREX_FORCE_INLINE
+    bool negative () noexcept
+    {
+        // the leftmost bit is our id sign
+        return m_idata >> 63;
+    }
 };
 
 struct ParticleCPUWrapper