Simplify constexp.pas.

I tried to fix #40777 the obvious way of modifying operator shl(a, b: Tconstexprint): Tconstexprint, and it indeed worked.

I also attempted to simplify some more of constexp.pas (−25% LoC!), using this idea in particular. But while checking against existing implementations I found two more examples of wrong trunk behavior, C2 and C3 below, where C3 is NOT fixed by this MR:

const
	C1 = 1 shl 63; // Trunk: silent −9223372036854775808. MR: correct 9223372036854775808.
	C2 = High(uint64) - 2 - High(uint64); // Trunk: “Overflow in arithmetic operation”, MR: correct −2.
	// C3 = High(int64) + 1 + -1; // “Overflow in arithmetic operation”, not fixed by the MR.

begin
{$if C1 < 0} {$error 1 shl 63 < 0} {$endif}
{$if C2 + 2 <> 0} {$error High(uint64) - 2 - High(uint64) <> -2} {$endif}
end.

C3 looks as if the compiler converts −1 to a qword during the typecheck (did it have to do that???????), and then the addition of two qwords fails. It hasn’t gotten any worse, anyway.

Update: literal $8000000000000000 is also considered negative, so I added an explicit conversion to tconstprop1.pp for now 😓.