[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>