[Bug tree-optimization/126710] missed ifcvt optimization with clz

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126710

Drea Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
            Summary|Missing CLZ detection       |missed ifcvt optimization
                   |                            |with clz
   Last reconfirmed|                            |2026-08-08
             Blocks|114527                      |
             Status|UNCONFIRMED                 |NEW

--- Comment #1 from Drea Pinski <pinskia at gcc dot gnu.org> ---
So currently clz is produced from sccp.

But what is produced is interesting.
```
  value_11 = value_4(D) >> 1;
  if (value_11 != 0)
    goto <bb 4>; [89.00%]
  else
    goto <bb 5>; [11.00%]

  <bb 4> [local count: 105119324]:
  _9 = .CLZ (value_11, 64);
  _13 = (unsigned int) _9;
  _10 = _13 + 4294967295;
  _1 = (int) _10;

  <bb 5> [local count: 150883496]:
  # _3 = PHI <64(2), 63(3), _1(4)>
```

A few things here, first this:
```
  _13 = (unsigned int) _9;
  _10 = _13 + 4294967295;
  _1 = (int) _10;
```
We should see _9 - 1 does not overflow and change this back into:
_1 = _9 + -1;

That leaves us with:
```
  t_4 = a_3(D) >> 1;
  if (t_4 != 0)
    goto <bb 3>; [48.89%]
  else
    goto <bb 4>; [51.11%]

  <bb 3> [local count: 524952376]:
  _1 = .CLZ (t_4, 32);
  _5 = _1 + -1;

  <bb 4> [local count: 1073741824]:
  # _2 = PHI <_5(3), 31(2)>
```
Which should be optimized to:
.CLZ (a_3, 32)
iff a_3 is non-zero.


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114527
[Bug 114527] [meta-bug] missed sccp (final value) optimizations