[Bug tree-optimization/115449] New: `(truncate)a` and `(nop_convert)~(truncate)a`

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115449

            Bug ID: 115449
           Summary: `(truncate)a` and `(nop_convert)~(truncate)a`
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Keywords: missed-optimization, TREE
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
  Target Milestone: ---

Take:
```
void setBit(unsigned char &a, int b) {
   unsigned char c = 0x1UL << b;
   a &= ~c;
    a |= c;
}

void setBit(signed char &a, int b) {
   signed char c = 0x1UL << b;
   a &= ~c;
    a |= c;
}
```

We should be able to optimize both setBit on the gimple level to:
```
  _1 = 1 << b_4(D);
  c_5 = ([un]signed char) _1;
  _2 = *a_7(D);
  _3 = _2 | c_5;
  *a_7(D) = _3;
```
Removing the ~ that is there.


So we already have this pattern (deals with some type changes though) in
gimple:
 /* (~x | y) & x -> x & y */
 /* (~x & y) | x -> x | y */
 (simplify
  (bitop:c (rbitop:c @2 @1) @0)
  (with { bool wascmp; }
   (if (bitwise_inverted_equal_p (@0, @2, wascmp)
        && (!wascmp || element_precision (type) == 1))
    (bitop @0 @1))))

The problem is bitwise_inverted_equal_p does not see that:
  c.0_4 = (signed char) _1;
  _5 = ~c.0_4;
  _16 = (charD.11) _5;

and
  c_11 = (charD.11) _1;

are bitwise inversions of each other.