[SimpleLoopUnswitch] unswitch selects

The old LoopUnswitch pass unswitched selects, but the changes were never
ported to the new SimpleLoopUnswitch.

We unswitch by turning:

```
S = select %cond, %a, %b
```

into:

```
head:
br %cond, label %then, label %tail

then:
br label %tail

tail:
S = phi [ %a, %then ], [ %b, %head ]
```

Unswitch selects are always nontrivial, since the successors do not exit
the loop and the loop body always needs to be cloned.

Differential Revision: https://reviews.llvm.org/D138526



Co-authored-by: Sergey Kachkov <sergey.kachkov@syntacore.com>