Add ParticleIDWrapper::flip_valid()

A cheaper way to swap validity sign on particle ids.
Not the same as `id = -id`, but also reversible.

Src/Particle/AMReX_Particle.H

@@ -102,6 +102,29 @@ struct ParticleIDWrapper
         r = (sign) ? lval : -lval;
         return r;
     }
+
+    /** Swap the the particle from invalid to valid or vice versa.
+     *
+     * Swaps the is_valid (sign) bit. This is NOT identical to id = -id,
+     * but it is equally reversible.
+     */
+    AMREX_GPU_HOST_DEVICE AMREX_FORCE_INLINE
+    void flip_valid () noexcept
+    {
+        // 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).
+     */
+    [[nodiscard]] AMREX_GPU_HOST_DEVICE AMREX_FORCE_INLINE
+    bool negative () const noexcept
+    {
+        // the leftmost bit is our id sign
+        return m_idata >> 63;
+    }
 };
 
 struct ParticleCPUWrapper
@@ -173,6 +196,17 @@ struct ConstParticleIDWrapper
         r = (sign) ? lval : -lval;
         return r;
     }
+
+    /** Check the sign of the id.
+     *
+     * Returns false if the id is positive, otherwise true (invalid particle).
+     */
+    [[nodiscard]] AMREX_GPU_HOST_DEVICE AMREX_FORCE_INLINE
+    bool negative () const noexcept
+    {
+        // the leftmost bit is our id's inverse sign
+        return !(m_idata >> 63);
+    }
 };
 
 struct ConstParticleCPUWrapper