[Bug middle-end/30314] optimize multiply-by-constant overflow (wrap) test

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=30314

--- Comment #6 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
So the rest is to fold the statements after widening pass.
I have a patch for that which does it in isel as I am going to have some isel
related match patterns.

Note gcc.target/i386/pr95852-2.c and gcc.target/i386/pr95852-4.c testcases need
to be fixed also since they have this pattern where widdening pass is the only
place which finds the _OVERFLOW variant and then it gets folded.

E.g.:
```
int
f17 (unsigned x)
{
  unsigned r = x * 35U;
  return x && (r / x) != 35U;
}
```