swift234

[overflow] Enable the pass the removes overflow checks by default.

We can now remove overflow checks by analyzing control flow. For example:

func foo(x : Int) {
  if x > 2 {
    x - 2 // overflow check removed
  }
}

We also remove overflow checks that are guarded by other (stronger) checks:

func bar(x : Int) {
  x + 1 // removed
  x + 5
  x + 2 // removed
}

With this change we are able to remove dozens of checks from the swift standard
library. However, only the only test in the test suite that becomes faster is
Fibonacci (1.3x).

Swift SVN r26054