Invalid branch code generation of type tests in VS 17.2.6

[miegir]

Steps to reproduce:

1. Launch VS 17.2.6 and create F# console application targeting .NET 6.
2. Replace contents Program.fs with the following code:

let classify (o: obj) =
    match o with
    | :? System.DateTime as d when d = Unchecked.defaultof<_> -> "default date"
    | :? System.IFormattable -> "formattable"
    | _ -> "not a date"
    
classify System.DateTime.Now
    |> printfn "%s"

Expected behavior: "formattable" should be printed to the console.
Actual behavior: "not a date" is printed to the console.

This is a regression, and the code is correctl executes at sharplab.io for now (see https://sharplab.io/#v2:DYLgZgzgNALiBOBXAdlAJiA1AH2AUxgAIBjYAQwggEswBPQgCgHsRCmAjAKwEpCBeALAAoQqMIBbMjGIALNoQDuVGDOFjC2QiAD8hAMq0IMPOIB0AESl4AKlXF5CFQmkUy8yZ/0IBVZLLzEANZ4aKZoeGBkiMAwTGAAPAD6AHyEALSpAEThkdFEaFaZamKaOvqGxmYAkgBiTPCSMDBk7PjpWWD1jc2teEUiJYSJ7YSZyExEZM6FxaLCpBTUdOVGJhZWtvamAHJMCrMaqQAO8FTIMGAemQCkEP1AA).

This code was also compiling correctly in VS earlier, but that "earlier" is several years ago and I do not know exactly when it was broken.

Incorrectly generated code in Release mode decompiles to the following C# code:

using System;

public static string classify(object o)
{
	if (o is DateTime)
	{
		DateTime d = default(DateTime);
		if (DateTime.Equals((DateTime)o, d))
		{
			d = (DateTime)o;
			return "default date";
		}
	}
	else if (o is IFormattable)
	{
		return "date";
	}
	return "not a date";
}

The compiler incorrectly assumes that DateTime does not implement IFormattable.

[vzarytovskii]

Reproduces in main.
@dsyme this looks related to #13178