memory align cause weird bug

[test123]

I has a program use with a bug cause me long time to find.

I use struct inside other struct, children struct has a memory 
pointer like this:

```d
struct Block {
    void* s;
}
```

And it can be put anywhere in parent struct, when use with fiber, 
some time it require member align with 16.


```d
auto diff =  (cast(size_t) &(context.block)) % (size_t.sizeof *2);
```

If the diff is 8,  I get error like this:

```sh
==32255==ERROR: AddressSanitizer: SEGV on unknown address (pc 
0x59808054d4d9 bp 0x716c2111ad10 sp 0x716c2111abf0 T0)
==32255==The signal is caused by a READ memory access.
==32255==Hint: this fault was caused by a dereference of a high 
value address (see register values below).  Disassemble the 
provided pc to learn which register was used.
```

I use 1.29.0-git-0b81277.

This is a new bug, and it work for my old ldc 1.28.1 on alpine.