[LV] Fix bug when unrolling (only) a loop with non-latch exit

If we unroll a loop in the vectorizer (without vectorizing), and the cost model requires a epilogue be generated for correctness, the code generation must actually do so.

The included test case on an unmodified opt will access memory one past the expected bound.  As a result, this patch is fixing a latent miscompile.

Differential Revision: https://reviews.llvm.org/D103700