swift

11525cab - [SILOptimizer]: slow OSSA lifetime canonicalization mitigation

[SILOptimizer]: slow OSSA lifetime canonicalization mitigation OSSA lifetime canonicalization can take a very long time in certain cases in which there are large basic blocks. to mitigate this, add logic to skip walking the liveness boundary for extending liveness to dead ends when there aren't any dead ends in the function. Updates `DeadEndBlocks` with a new `isEmpty` method and cache to determine if there are any dead-end blocks in a given function. (cherry picked from commit 1f3f830fc7aa36b1adba00c251b9c24a202c2af2)