[Bug tree-optimization/19832] don't remove an if when we know the value is the same as with the if (subtraction)

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=19832

--- Comment #8 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Xor should be handled too:
```
int f_xor(int i, int j)
{
  if (i!=j)
    return i ^ j;
  return 0;
}
``

ior and and should be handled
```
int f_or(int i, int j)
{
  if (i!=j)
    return i | j;
  return i; // could be j not just i
}

int f_and(int i, int j)
{
  if (i!=j)
    return i & j;
  return i; // could be j not just i
}
```

So can plus and multiply:
```
int f_add(int i, int j)
{
  if (i!=j)
    return i + j;
  return i+i;
}
int f_mult(int i, int j)
{
  if (i!=j)
    return i * j;
  return i*i;
}
```


Note clang handles all of these except for f_add. f_mult might be handled via
the pull `i*` out of the conditional and then you have `i!=j?j:i` which then
will be reduced to j (that is they don't pattern match f_mult). They don't have
pattern matching for f_add either and `i+i` will change to `i*2` and not pulled
out of the condition.