Better translation for COUNT with predicate checking for non-null

[roji]

We currently translate the following LINQ query:

_ = ctx.Blogs
    .GroupBy(b => b.Id)
    .Select(g => new { g.Key, Count = g.Select(x => x.Name).Count(n => n != null) })
    .ToList();

... to:

SELECT [b].[Id] AS [Key], COUNT(CASE
    WHEN [b].[Name] IS NOT NULL THEN 1
END) AS [Count]
FROM [Blogs] AS [b]
GROUP BY [b].[Id]

... when we could just translate to:

SELECT [b].[Id] AS [Key], COUNT([b].[Name]) AS [Count]
FROM [Blogs] AS [b]
GROUP BY [b].[Id]

... since COUNT(x) only counts non-null values. This may have a perf impact (index usage).

Related to #26938