[runtimeunroll] Support epilogue unrolling with a parent loop

This patch adds support for unrolling inner loops using epilogue unrolling. The basic issue is that the original latch exit block of the inner loop could be outside the outer loop.  When we clone the inner loop and split the latch exit, the cloned blocks need to be in the outer loop.

Differential Revision: https://reviews.llvm.org/D108476