[Bug tree-optimization/126763] New: a?__builtin_clz(a):__builtin_clz(31) -> clz(a|1)

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126763

            Bug ID: 126763
           Summary: a?__builtin_clz(a):__builtin_clz(31) -> clz(a|1)
           Product: gcc
           Version: 17.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
  Target Milestone: ---

Take:
```
int f(unsigned a)
{
    if (a)
      return __builtin_clz(a);
    return __builtin_clz(1);
}

int f0(unsigned a)
{
    return __builtin_clz(a|1);
}
int n(unsigned a)
{
    return __builtin_clzg(a,__builtin_clz(1)+1);
}
```

This all have well defined code and all return the same thing. f0 produces the
best results; 31 (assuming `sizeof(int)==4`).

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114032#c3 mentions we could add
support for CLZ for other than zero values than CLZ_VALUE_DEFINED_AT_ZERO, that
might be a good idea. For 31 (or 63) we could special case that in expand as
being `a|1` which should be able to support the value always.