Don't loop unswitch vector selects

Otherwise we could produce `br <2x i1>` which are of course not legal.

```
Branch condition is not 'i1' type!
  br <2 x i1> %cond.fr1, label %entry.split.us, label %entry.split
  %cond.fr1 = freeze <2 x i1> %cond
LLVM ERROR: Broken module found, compilation aborted!
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace.
Stack dump:
0.	Program arguments: /home/vchuravy/builds/llvm/bin/opt -passes=simple-loop-unswitch<nontrivial> -S
```

Fixes change introduced by https://reviews.llvm.org/D138526

Reviewed By: caojoshua

Differential Revision: https://reviews.llvm.org/D149560