[LoopUnroll] Avoid branch on poison for runtime unroll with multiple exits

When performing runtime unrolling with multiple exits, one of the
earlier (non-latch) exits may exit the loop on the first iteration,
such that we never branch on the latch exit condition. As such, we
need to freeze the condition of the new branch that is introduced
before the loop, as it now executes unconditionally.

Differential Revision: https://reviews.llvm.org/D125754