[Bug tree-optimization/122845] New: `a ^ CST ^ CST` is sometimes not folded at the gimple level (casts)

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122845

            Bug ID: 122845
           Summary: `a ^ CST ^ CST` is sometimes not folded at the gimple
                    level (casts)
           Product: gcc
           Version: 16.0
            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:
```
struct s1
{
  signed f1:3;
};
struct s1 p;
void f(void)
{
 p.f1 ^= -47;
 p.f1 ^= -47;
}
```

This should be simplified into a nop but currently it is not done on the gimple
level due to casts.

We get:
```
  _1 = p.f1;
  _2 = (signed char) _1;
  _3 = _2 ^ -47;
  _4 = (<unnamed-signed:3>) _3;
  _5 = (signed char) _4;
  _6 = _5 ^ -47;
  _7 = (<unnamed-signed:3>) _6;
  p.f1 = _7;
```

This is a side issue reduced from PR 122843.