Fix #1144: Inline away for-ranges

The first 3 commits add a few simple improvements to the IR and optimizer.

The 4th commit adds insane optimizations for `Long`s, getting rid of as many of them as possible. There's even one case that says that
`e1.toLong + e2.toLong > 2147483647L` (`Int.MaxValue.toLong`),
where `e1` and `e2` are `Int`s, is equivalent to:
```scala
{
  val x = e1
  val y = e2
  (x > 0 && y > 0 && x+y < 0)
}
```

The last commits actually change `Range` to make it inlineable.
Also "is valid" doesn't very well describe "doesn't overflow"...