Summary:
LoopRotate manually updates the DoomTree by iterating over all predecessors of a basic block and computing the Nearest Common Dominator.
When a predecessor happens to be unreachable, `DT.findNearestCommonDominator` returns nullptr.
This patch teaches LoopRotate to handle this case and fixes [[ https://bugs.llvm.org/show_bug.cgi?id=33701 | PR33701 ]].
In the future, LoopRotate should be taught to use the new incremental API for updating the DomTree.
Reviewers: dberlin, davide, uabelho, grosser
Subscribers: efriedma, mzolotukhin
Differential Revision: https://reviews.llvm.org/D35074
llvm-svn: 307828
Jakub Kuderski