[VFX] Incorrect mesh rendered when using multimesh output and exposed submesh mask

When implementing instancing support for exposed meshes and textures, we missed one specific case.
If we use a multimesh output with a exposed submesh mask that comes from a expression tree that can be optimized, the expression will not be registered properly in runtime mode.
if the VFX is compiled in edition mode, the issue will not be visible.

Author: gabrieldelacruz

Packages/com.unity.visualeffectgraph/Editor/Data/VFXDataParticle.cs

@@ -1368,8 +1368,9 @@ int GetBufferIndex(VFXTask task, string baseName)
                         VFXEditorTaskDesc singleMeshTaskDesc = taskDesc;
                         singleMeshTaskDesc.parameters = VFXMultiMeshHelper.PatchCPUMapping(taskDesc.parameters, multiMeshOutput.meshCount, j).ToArray();
                         singleMeshTaskDesc.buffers = VFXMultiMeshHelper.PatchBufferMapping(taskDesc.buffers, j).ToArray();
-                        VFXMultiMeshHelper.PatchInstancingSplitValues(instancingSplitDescValues, expressionGraph, context.inputSlots, multiMeshOutput.meshCount, j);
-                        singleMeshTaskDesc.instanceSplitIndex = AddInstanceSplitDesc(instanceSplitDescs, instancingSplitDescValues);
+                        var instancingSplitDescValuesMesh = new List<uint>(instancingSplitDescValues);
+                        VFXMultiMeshHelper.PatchInstancingSplitValues(instancingSplitDescValuesMesh, expressionGraph, context.inputSlots, multiMeshOutput.meshCount, j);
+                        singleMeshTaskDesc.instanceSplitIndex = AddInstanceSplitDesc(instanceSplitDescs, instancingSplitDescValuesMesh);
                         AddTaskDesc(taskDescs, singleMeshTaskDesc, context);
                     }
                 }

Packages/com.unity.visualeffectgraph/Editor/Models/Contexts/VFXMultiMeshHelper.cs

@@ -103,6 +103,9 @@ public static void PatchInstancingSplitValues(List<uint> instancingSplitDescValu
                 var exp = inputSlots.First(s => s.name == name).GetExpression();
                 if (exp != null && !exp.IsAny(VFXExpression.Flags.Constant))
                 {
+                    // Obtain reduced version of the expression
+                    exp = expressionGraph.CPUExpressionsToReduced.First(kvp => kvp.Key == exp).Value;
+
                     int index = expressionGraph.GetFlattenedIndex(exp);
                     if (index >= 0)
                     {