[SimpleLoopUnswitch] Skip trivial select conds for selects

Fixes https://github.com/llvm/llvm-project/issues/62715

If a select's condition is a trivial select:
```
%s = select %cond, i1 true, i1 false
```
Unswitch on %cond, rather than %s. This fixes crashes where there is a
disparity in finding candidates and and the transformation logic.