[LoopUnroll] Peel iterations based on select conditions

This also allows us to peel loops with a `select`:
```
for (int i = 0; i <= N; ++i);
  f3(i == 0 ? a : b); // select instruction
```
into:
```
f3(a); // peel one iteration
for (int i = 1; i <= N; ++i)
  f3(b);
```

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D151052