[Bug c++/104588] New: user-defined constructor loses alignment information about `*this`

["lh_mouse at 126 dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104588

            Bug ID: 104588
           Summary: user-defined constructor loses alignment information
                    about `*this`
           Product: gcc
           Version: 11.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: lh_mouse at 126 dot com
  Target Milestone: ---
            Target: x86_64-linux-gnu

https://gcc.godbolt.org/z/M3YoaYeEf

```c++
struct alignas(16) foo
  {
    unsigned char a[32];
    foo() : a() { }
  };t

struct alignas(16) bar
  {
    foo f;
    long m,n,p,q;
    bar() : f(),m(),n(),p(),q() { }
  };

constexpr void* operator new(unsigned long, void* p) noexcept { return p;  }
inline void operator delete(void*, void*) noexcept { }

bar* construct(bar* p)
  {
    return new(p) bar();
  }
```


For `bar::bar()` with -O3, GCC generates two `MOVUPS` followed by two `MOVAPS`,
while Clang generates four `MOVAPS`. `MOVUPS` was slow on some older CPUs.


GCC output:

```
construct(bar*):
        pxor    xmm0, xmm0
        mov     rax, rdi
        movups  XMMWORD PTR [rdi], xmm0
        movups  XMMWORD PTR [rdi+16], xmm0
        pxor    xmm0, xmm0
        movaps  XMMWORD PTR [rdi+32], xmm0
        movaps  XMMWORD PTR [rdi+48], xmm0
        ret
```

Clang output:

```
construct(bar*):                      # @construct(bar*)
        mov     rax, rdi
        xorps   xmm0, xmm0
        movaps  xmmword ptr [rdi + 48], xmm0
        movaps  xmmword ptr [rdi + 32], xmm0
        movaps  xmmword ptr [rdi + 16], xmm0
        movaps  xmmword ptr [rdi], xmm0
        ret
```