Materializer optimizations

[roji]

As part of the AOT work (#25009), we now have a (hopefully) 100% faithful C# representation of the materializer expression tree (see code below).

Here are some quick perf fixes we can do:

• Remove the materializationContext creation when not needed:

var materializationContext1 = new MaterializationContext(ValueBuffer.Empty, queryContext0.Context);

• Eliminate the object array allocation and the boxing for non-composite key:

var entry1 = queryContext0.TryGetEntry(key, new object[]{(object)dataReader.GetInt32(0)}, true, out hasNullKey1);

• Avoid unneeded always-true entity type check:

entityType1 = entityType;
if (entityType1 == entityType)

• Avoid check for null entity type (when is this possible?):

entry1 = entityType1 == default(IEntityType) ? default(InternalEntityEntry) : queryContext0.StartTracking(entityType1, instance1, shadowValueBuffer1);

• Avoid explicitly setting instance to null, as it's already been initialized to null above:

else
{
    instance1 = null;
}

• Query: Be smart about what to store in variables for shaper #21336
• Query: Inline shapers for the cases when we don't need variables #21334

Code

IEntityType entityType1;
bool hasNullKey1;
var materializationContext1 = new MaterializationContext(ValueBuffer.Empty, queryContext0.Context);
Blog instance1 = null;
var entry1 = queryContext0.TryGetEntry(key, new object[]{(object)dataReader.GetInt32(0)}, true, out hasNullKey1);
if (!hasNullKey1)
{
    if (entry1 != default(InternalEntityEntry))
    {
        entityType1 = entry1.EntityType;
        instance1 = (Blog)entry1.Entity;
    }
    else
    {
        var shadowValueBuffer1 = ValueBuffer.Empty;
        entityType1 = entityType;
        if (entityType1 == entityType)
        {
            var instance0 = Activator.CreateInstance<Blog>();
            typeof(Blog).GetField("<Id>k__BackingField", BindingFlags.Instance | BindingFlags.NonPublic).SetValue(instance0, dataReader.GetInt32(0));
            typeof(Blog).GetField("<Name>k__BackingField", BindingFlags.Instance | BindingFlags.NonPublic).SetValue(instance0, dataReader.GetString(1));
            instance1 = instance0;
        }
        else
        {
            instance1 = null;
        }

        entry1 = entityType1 == default(IEntityType) ? default(InternalEntityEntry) : queryContext0.StartTracking(entityType1, instance1, shadowValueBuffer1);
    }
}

var entity = instance1;
return entity;