Inlined ‘Self’-changing type helpers over values accessed by dereference either do not work or cause internal errors.

While trying to fix #39841 (closed) (by changing something randomly and seeing if it works) I tried to compile tests/webtbs/tw38122.pas and got an ICE on the line 72:

px^.sub(100);  //and this works as well.

Then I remembered I had -OoAUTOINLINE, but quickly found an offending function whose manual inlining also fails perfectly, and built a simpler example:

{$mode objfpc} {$typedaddress on} {$modeswitch typehelpers}
type
	XType = double;

	XTypeHelper = type helper for XType
		procedure Add(x: XType); inline;
	end;

	procedure XTypeHelper.Add(x: XType);
	begin
		self += x;
	end;

var
	x: XType;

begin
	x := 0;
	(@x)^.Add(1);
	if x <> 1 then begin writeln('x = ', x, ', expected 1'); halt(1); end;
	writeln('ok');
end.

Compiling this for x86_64-win64 with floating-point XType gives Internal error 200109227.

Compiling this for i386-win32 with floating-point XType gives Internal error 200306031.

Compiling this with integral (or even record, with corresponding modifications) XType proceeds, but running fails:

x = 0, expected 1

Probably related and can be fixed (one day) together with #39841 (closed), as my #38122 (closed)-reintroducing patch fixes this as well.