Fix Issue 21424 - Variable is incremented twice

[BorisCarvajal]

Test case:

    ubyte[10] buf;
    size_t pos = 0;
    size_t num = 5;
    buf[pos++] += num;

Here DMD internally generates something like:
cast(ulong)cast(int) (tmp = pos++ + &buf, *tmp) += num

That expression had to be converted to:
(tmp = pos++ + &buf, cast(ulong)cast(int) *tmp) += num
but the implementation worked only for one cast (fixed now). And because of that the compiler ended up creating an assign operation duplicating the comma exp in both sides:
(tmp = pos++ + &buf, *tmp) = cast(...)(tmp = pos++ + &buf, *tmp) + num

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	21424	normal	Variable is incremented twice

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#11997"

[thewilsonator]

if (econv.EV.E1.Eoper != OPcomma)
{
    econv = econv.EV.E1;
    continue;
}

[BorisCarvajal]

done.

[thewilsonator]

Otherwise looks good