Prefer equality when comparing values

[ranma42]

Most databases can take advantage of indexes (and in some cases even auto-generate them) when queries perform equality comparisons (instead of inequality).

In some places EFCore already tries to lean towards equality

efcore/src/EFCore.Relational/Query/SqlExpressionFactory.cs

Lines 657 to 666 in 20edb63

	// use equality where possible
	// !(a == true) -> a == false
	// !(a == false) -> a == true
	SqlBinaryExpression { OperatorType: ExpressionType.Equal, Right: SqlConstantExpression { Value: bool } } binary
	=> Equal(binary.Left, Not(binary.Right)),
	// !(true == a) -> false == a
	// !(false == a) -> true == a
	SqlBinaryExpression { OperatorType: ExpressionType.Equal, Left: SqlConstantExpression { Value: bool } } binary
	=> Equal(Not(binary.Left), binary.Right),

In other places it disregards this and in fact can even replace equality comparisons with inequalities

efcore/src/EFCore.Relational/Query/SqlNullabilityProcessor.cs

Lines 1748 to 1757 in 20edb63

	// a == b <=> !a == !b -> a == b
	// !a == b <=> a == !b -> a != b
	// a != b <=> !a != !b -> a != b
	// !a != b <=> a != !b -> a == b
	nullable = false;
	return sqlBinaryExpression.OperatorType == ExpressionType.Equal ^ leftNegated == rightNegated
	? _sqlExpressionFactory.NotEqual(left, right)
	: _sqlExpressionFactory.Equal(left, right);

efcore/src/EFCore.Relational/Query/SqlNullabilityProcessor.cs

Lines 1803 to 1804 in 20edb63

	// a == !b and !a == b in SQL evaluate the same as a != b
	body = _sqlExpressionFactory.NotEqual(left, right);

It would be better to avoid converting equalities into inequalities and, when possible, convert inequalities into equalities.