Add helper function for setting id and cpu simultaneously. (#3733)

Also sneaking in a small ignore_unused fix.

Close #3731.

Co-authored-by: Axel Huebl <axel.huebl@plasma.ninja>

Src/Particle/AMReX_Particle.H

@@ -188,6 +188,18 @@ struct ConstParticleCPUWrapper
     operator int () const noexcept { return static_cast<int>(m_idata & 0x00FFFFFF); }
 };
 
+/** Set the idcpu value at once, based on a particle id and cpuid
+ *
+ * This can be used in initialization and assignments,
+ * to avoid writing twice into the same memory bank.
+ */
+AMREX_GPU_HOST_DEVICE AMREX_FORCE_INLINE
+std::uint64_t SetParticleIDandCPU (Long id, int cpu) noexcept{
+    std::uint64_t idcpu = 0;
+    ParticleIDWrapper{idcpu} = id;
+    ParticleCPUWrapper{idcpu} = cpu;
+    return idcpu;
+}
 
 template <typename T, int NReal, int NInt>
 struct ParticleBase

Src/Particle/AMReX_ParticleIO.H

@@ -1094,6 +1094,7 @@ ParticleContainer_impl<ParticleType, NArrayReal, NArrayInt, Allocator, CellAssig
             {
                 new_size += src_tile.size();
             } else {
+                amrex::ignore_unused(src_tile);
                 new_size += host_real_attribs[host_lev][std::make_pair(grid,tile)][0].size();
             }
             dst_tile.resize(new_size);