[Bug rtl-optimization/126726] New: smax(a,-a) -> abs for RTL

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126726

            Bug ID: 126726
           Summary: smax(a,-a) -> abs for RTL
           Product: gcc
           Version: 17.0
            Status: UNCONFIRMED
          Keywords: easyhack, missed-optimization
          Severity: enhancement
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
  Target Milestone: ---

Similar to PR 55152 but this time for the RTL level; sometimes smax shows up
during RTL.
Take:
```
int __GIMPLE (ssa,startwith("optimized"))
f (int a)
{
  int t;
  int _3;

  __BB(2):
  t_2 = -a_1(D);
  _3 = __MAX (t_2, a_1(D));
  return _3;

}
```
On aarch64 with -O2 -march=armv9.5-a (enabling CSSC) we get:
```
f:
        neg     w1, w0
        smax    w0, w1, w0
        ret
```

Note I noticed this while looking into Wren6991's code changes to add a
peephole but really this should be done in simplify-rtl.cc.

https://github.com/Wren6991/gcc/commit/2ec0d1a0ba822a539fd26261707941c357be9147