[LoopUnswitch] Unswitch on conditions feeding into guards

Summary:
This is a straightforward extension of what LoopUnswitch does to
branches to guards.  That is, we unswitch

```
for (;;) {
  ...
  guard(loop_invariant_cond);
  ...
}
```

into

```
if (loop_invariant_cond) {
  for (;;) {
    ...
    // There is no need to emit guard(true)
    ...
  }
} else {
  for (;;) {
    ...
    guard(false);
    // SimplifyCFG will clean this up by adding an
    // unreachable after the guard(false)
    ...
  }
}
```

Reviewers: majnemer

Subscribers: mcrosier, llvm-commits, mzolotukhin

Differential Revision: http://reviews.llvm.org/D21725

llvm-svn: 273801