[Bug tree-optimization/112392] New: `a == nonnegative ? a : abs` -> abs

[pinskia at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112392

            Bug ID: 112392
           Summary: `a == nonnegative ? a : abs<a>` -> abs<a>
           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(int a, unsigned char b)
{
  int absb = b;
  if (a == absb)  return absb;
  return a > 0 ? a : -a;
}
```
This should just be turned into
`return ABS_EXPR<a>;`
as we know that if a was a nonnegative value, then taking the ABS of that is
the same as a.

So something like:
```
(for cmp (eq gt ge)
 (simplify
  (cond (cmp:c @0 tree_expr_nonnegative_p@1) @0 (abs@3 @0))
  (if (INTEGRAL_TYPE_P (type))
   @3))
```

`a > nonnegative ? a : abs<a>` is also just `abs<a>` too. 

There are some more improvements dealing with type changing between signed and
unsigned but this is the first step.