LoopRotate: don't rotate loops where the header block branches to a dead-end block.

[eeckstein]

Incomplete liveranges in the dead-end exit block can cause a missing adjacent phi-argument for a re-borrow if there is a borrow-scope is in the loop. But even when we have complete lifetimes, it's probably not worth rotating a loop where the header block branches to a dead-end block.

Fixes a SIL verifier crash

[eeckstein]

@swift-ci test

[eeckstein]

@swift-ci apple silicon benchmark

[meg-gupta]

lgtm

[atrick]

it's probably not worth rotating a loop where the header block branches to a dead-end block.

I thought that was actually the primary use case for loop rotation--making sure the trap condition dominates the loop.

[eeckstein]

@swift-ci smoke test linux

[eeckstein]

I thought that was actually the primary use case for loop rotation--making sure the trap condition dominates the loop.

That's a good point. But usually trap conditions are optimized to non-control flow cond_fail instructions. So loop rotation does not consider those anyway.
It just can happen some times that a cond_br - cond_fail true pair is not optimized yet when loop rotation runs.

I think we want another kind of optimization which moves cond_fail instructions out of the loop for the purpose of removing them in the loop.