`__can_pass_in_regs(very-big-class)` wrong on x86-64

https://godbolt.org/z/qK33K7ahc

```cpp
struct A {
long data[1024*1024];
};

static_assert(__can_pass_in_regs(A));
```

x86-64 psABI:
> MEMORY This class consists of types that will be passed and returned in memory via the stack.
> ...
> If the size of an object is larger than eight eightbytes, or it contains unaligned fields, it has class MEMORY

Docs: https://clang.llvm.org/docs/LanguageExtensions.html#:~:text=%5F%5Fcan%5Fpass%5Fin%5Fregs

Reproduce the static assertion in the linked Godbolt example and read the __can_pass_in_regs documentation alongside the x86-64 psABI classification quoted in the issue. Trace how the builtin handles a very large class, then verify that a class exceeding eight eightbytes is treated as MEMORY and the assertion no longer passes.

Written by the indexing model from the issue text.