[LoopUnroll] Fix block frequencies for epilogue (#159163) As another step in issue #135812, this patch fixes block frequencies for partial loop unrolling with an epilogue remainder loop. It does not fully handle the case when the epilogue loop itself is unrolled. That will be handled in the next patch. For the guard and latch of each of the unrolled loop and epilogue loop, this patch sets branch weights derived directly from the original loop latch branch weights. The total frequency of the original loop body, summed across all its occurrences in the unrolled loop and epilogue loop, is the same as in the original loop. This patch also sets `llvm.loop.estimated_trip_count` for the epilogue loop instead of relying on the epilogue's latch branch weights to imply it. This patch fixes branch weights in tests that PR #157754 adversely affected.
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