[Bug target/126973] New: Worse X86 codegen for arbitrary __builtin_clzg/__builtin_ctzg compared to clang.

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126973

            Bug ID: 126973
           Summary: Worse X86 codegen for arbitrary
                    __builtin_clzg/__builtin_ctzg compared to clang.
           Product: gcc
           Version: 17.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kaelfandrew at gmail dot com
  Target Milestone: ---

https://godbolt.org/z/zTrYKvvxK

```
#define N 16

int
clzg_uN (unsigned _BitInt(N) x)
{
  return __builtin_clzg (x);
}

int
ctzg_uN (unsigned _BitInt(N) x)
{
  return __builtin_ctzg (x);
}
```

With -O2 and mostly N <= 64, Clang produce much better code:

N = 16
``` GCC
"clzg_uN":
        movzx   edi, di
        bsr     eax, edi
        xor     eax, 31
        sub     eax, 16
        ret
"ctzg_uN":
        movzx   eax, di
        rep bsf eax, eax
        ret
```

``` Clang
clzg_uN:
        bsr     ax, di
        xor     eax, 15
        movzx   eax, ax
        ret

ctzg_uN:
        rep       bsf eax, edi
        ret
```

N = 37
```GCC
"clzg_uN":
        movabs  rax, 137438953471
        and     rdi, rax
        bsr     rax, rdi
        xor     rax, 63
        sub     eax, 27
        ret
"ctzg_uN":
        movabs  rax, 137438953471
        and     rdi, rax
        xor     eax, eax
        rep bsf rax, rdi
        ret
```

```Clang
clzg_uN:
        shl     rdi, 27
        bsr     rax, rdi
        xor     eax, 63
        ret

ctzg_uN:
        rep       bsf rax, rdi
        ret
```