Optimize away Coalesce for trivial cases

[ranma42]

This changeset contains:

• improvements to the test suite that ensure that COALESCE is not optimized away where it is the actual target of the test
• a change to the ISqlExpressionFactory interface that grants its members more flexibility on the instance type of the returned expressions; even though it is technically a breaking change, this only has impact on providers (and even then, the expected impact is limited, as seen in the changes required on the SQLite and SqlServer providers)
• an improvement to the SqlExpressionFactory.Coalesce implementation that automatically simplifies the expression for trivial cases:
  • COALESCE(null, x) -> x
  • COALESCE(nonNullableConstant, x) -> nonNullableConstant
  • COALESCE(nonNullableColumn, x) -> nonNullableColumn

Contributes to #33890 (the Cosmos part is missing).

[ranma42]

Note that this change only takes care of trivial optimizations (which is nonetheless convenient to centralize, as shown by the commit that cleans up repeated copies of this pattern).

More general optimizations for COALESCE are being implemented in:

• Simplify COALESCE based on the nullability of its arguments #33938

[ranma42]

we could also move this simplification to the factory, if we want to:

efcore/src/EFCore.Relational/Query/Internal/SqlExpressionSimplifyingExpressionVisitor.cs

Lines 65 to 93 in d6fd3d2

	if (extensionExpression is SqlFunctionExpression sqlFunctionExpression
	&& IsCoalesce(sqlFunctionExpression))
	{
	var arguments = new List<SqlExpression>();
	foreach (var argument in sqlFunctionExpression.Arguments!)
	{
	var newArgument = (SqlExpression)Visit(argument);
	if (IsCoalesce(newArgument))
	{
	arguments.AddRange(((SqlFunctionExpression)newArgument).Arguments!);
	}
	else
	{
	arguments.Add(newArgument);
	}
	}
	var distinctArguments = arguments.Distinct().ToList();
	return distinctArguments.Count > 1
	? new SqlFunctionExpression(
	sqlFunctionExpression.Name,
	distinctArguments,
	sqlFunctionExpression.IsNullable,
	argumentsPropagateNullability: distinctArguments.Select(_ => false).ToArray(),
	sqlFunctionExpression.Type,
	sqlFunctionExpression.TypeMapping)
	: distinctArguments[0];
	}

(aka automatically flatten COALESCE and deduplicate its arguments)

[ranma42]

The first 2 commits of this PR now also belong to:

• Improve some null-related tests #34019

hence I am marking this as draft like the other dependent PRs I posted 😇

[ranma42]

@cincuranet sorry, I did not notice that another change was merged that was not a syntax conflict, but that still resulted in a build issue. I just pushed #34078 to fix this.