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[LoopDeletion] Check for irreducible cycles when deleting loops.

Authored by fhahn on Jun 15 2021, 4:53 AM.

Description

[LoopDeletion] Check for irreducible cycles when deleting loops.

Loops with irreducible cycles may loop infinitely. Those cannot be
removed, unless the loop/function is marked as mustprogress.

Also discussed in D103382.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D104238