[Bug tree-optimization/114047] New: `(cast)(cast)a != a` for large unsigned _BitInt should be produce better code

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114047

            Bug ID: 114047
           Summary: `(cast)(cast)a != a` for large unsigned _BitInt should
                    be produce better code
           Product: gcc
           Version: 14.0
            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:
```
unsigned
foo (unsigned _BitInt(512) ub512_0)
{
  unsigned _BitInt(255) t;
  return __builtin_sub_overflow (ub512_0, 0, &t);
}

unsigned
foo1 (unsigned _BitInt(512) ub512_0)
{
  unsigned _BitInt(255) t = ub512_0;
  return t != ub512_0;
}
```

Currently foo produces decent code but foo1 does not even though they are
equivalent.

Noticed while looking into PR 114040.