FR: Fold floating-point subtractions under FASTMATH.
While trying to understand what #39782 wanted, I noticed that these examples indeed don’t fold.
```pascal
{$mode objfpc} {$h+}
{$optimization level1} {$optimization level2} {$optimization level3} {$optimization level4} {$optimization regvar} {$optimization fastmath}
label A0, A1, B0, B1, C0, C1;
var
x, y: single;
dontDrop: string;
begin
x := random;
A0:
y := x + 1 + 2 + 3 + 4 + 5 + 6; // folds
A1:
writestr(dontDrop, y);
writeln('x + 1 + 2 + 3 + 4 + 5 + 6: ', CodePointer(@A1) - CodePointer(@A0), ' b');
B0:
y := x + 1 - 2 + 3 - 4 + 5 - 6; // didn't fold at the time of making the issue, now folds
B1:
writestr(dontDrop, y);
writeln('x + 1 - 2 + 3 - 4 + 5 - 6: ', CodePointer(@B1) - CodePointer(@B0), ' b');
C0:
y := x - 1 - 2 - 3 - 4 - 5 - 6; // didn't fold at the time of making the issue, now folds
C1:
writestr(dontDrop, y);
writeln('x - 1 - 2 - 3 - 4 - 5 - 6: ', CodePointer(@C1) - CodePointer(@C0), ' b');
end.
```
⇓
`x + 1 + 2 + 3 + 4 + 5 + 6: 15 b`<br>
`x + 1 - 2 + 3 - 4 + 5 - 6: 55 b` (15 b since fcb5531fbc90cab7e998ce8eab655182d165ac50)<br>
`x - 1 - 2 - 3 - 4 - 5 - 6: 55 b` (15 b since 94665a40d760b6cdc22b89278c201ace10a0b57f)
issue