[Bug tree-optimization/109722] New: ABSU<a> == 0 is not optimized to just a == 0

["pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109722

            Bug ID: 109722
           Summary: ABSU<a> == 0 is not optimized to just a == 0
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
          Assignee: pinskia at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
  Target Milestone: ---

Take:
```
int f(signed char a)
{
        int t = a;
        if (t < 0) t = -t;
        a = t;
        return a == 0;
}
```
Right now this gets optimized to just:
```
  _9 = ABSU_EXPR <a_3(D)>;
  _1 = _9 == 0;
  _7 = (int) _1;
```

If we change `signed char` to `unsigned char`, we get:
  _1 = a_2(D) == 0;
  _3 = (int) _1;

As we had expected for the original case too.

The fix is to change in match.pd:
/* Convert ABS_EXPR<x> == 0 or ABS_EXPR<x> != 0 to x == 0 or x != 0.  */
(for op (eq ne)
 (simplify
  (op (abs @0) zerop@1)
  (op @0 @1)))
to:
/* Convert ABS[U]_EXPR<x> == 0 or ABS[U]_EXPR<x> != 0 to x == 0 or x != 0.  */
(for op (abs absu)
 (for eqne (eq ne)
  (simplify
   (eqne (op @0) zerop@1)
   (eqne @0 @1)))