[Bug tree-optimization/127706] Missed vector optimization: (a - b) != 0 is not simplified to a != b with an extra cast involved

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=127706

--- Comment #3 from Drea Pinski <pinskia at gcc dot gnu.org> ---
I should mention this works:
```
    typedef unsigned int v8u32 __attribute__((vector_size(32)));
    typedef int v8i32 __attribute__((vector_size(32)));

    __attribute__((noipa)) v8i32
    f (v8u32 a, v8u32 b)
    {
      v8u32 d = a - b;
      return (v8u32) d != (v8u32) { 0, 0, 0, 0, 0, 0, 0, 0 };
    }
```

Because this pattern works already for vectors:
```
/* Transform comparisons of the form X - Y CMP 0 to X CMP Y.
   ??? The transformation is valid for the other operators if overflow
   is undefined for the type, but performing it here badly interacts
   with the transformation in fold_cond_expr_with_comparison which
   attempts to synthetize ABS_EXPR.  */
(for cmp (eq ne)
 (for sub (minus pointer_diff)
  (simplify
   (cmp (sub@2 @0 @1) integer_zerop)
   (if (single_use (@2))
    (cmp @0 @1)))))
```