[Bug ipa/114262] Over-inlining when optimizing for size with gnu_inline function

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114262

--- Comment #4 from LIU Hao <lh_mouse at 126 dot com> ---
(In reply to Andrew Pinski from comment #3)
> It looks like it has been this way since r0-37737-g4838c5ee553f06 (2001) (or
> rather that is when it was used by the tree inline; I don't want to dig
> further back to understand the RTL inliner). So looks like this is just
> missing documentation ...

It's not just about `gnu_inline`. If we switch to C++ inline we get the same
result:

(https://gcc.godbolt.org/z/ehbjqj5xh)
```
struct impl;
struct impl* get_impl(int key);
int get_value(struct impl* p);


extern inline
int get_value_by_key(int key)
  {
    struct impl* p = get_impl(key);
    if(!p)
      return -1;
    return get_value(p);
  }

int real_get_value_by_key(int key)
  {
    return get_value_by_key(key);
  }
```

GCC outputs:
```
real_get_value_by_key(int):
        push    rsi
        call    get_impl(int)
        test    rax, rax
        je      .L2
        mov     rdi, rax
        pop     rcx
        jmp     get_value(impl*)
.L2:
        or      eax, -1
        pop     rdx
        ret
```


If we switched to C99 `extern inline` then it would produce desired result:
```
get_value_by_key:
        push    rsi
        call    get_impl
        test    rax, rax
        je      .L2
        mov     rdi, rax
        pop     rcx
        jmp     get_value
.L2:
        or      eax, -1
        pop     rdx
        ret
real_get_value_by_key:
        jmp     get_value_by_key
``

The only difference between the C99 `extern inline` and C++ `extern inline` is
that the C++ external definition is COMDAT.