[Bug tree-optimization/126396] New: (~a)>>a to -1 for vector types

[kaelfandrew at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126396

            Bug ID: 126396
           Summary: (~a)>>a to -1 for vector types
           Product: gcc
           Version: 17.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kaelfandrew at gmail dot com
  Target Milestone: ---

https://godbolt.org/z/6KjE8Wh7e

With -O2, src () does not optimize to tgt ():

```
typedef int v4si __attribute__ ((vector_size (16)));

v4si
src (v4si a)
{
  return (~a >> a);
}

v4si
tgt (void)
{
  v4si a = {-1, -1, -1, -1};
  return a;
}
```

Testcases should be vector versions of PR125707 and PR125790.

gcc/match.pd:

```
/* (~X) >> (type1)X -> -1 for signed X.
   The cast needs not to be truncating cast.  */
(simplify
 (rshift (bit_not @0) (convert?@1 @0))
 (if (INTEGRAL_TYPE_P (type)
      && !TYPE_UNSIGNED (type)
      && TYPE_PRECISION (type) <= TYPE_PRECISION (TREE_TYPE (@1)))
  { build_minus_one_cst (type); }))
```