CFG with cycles may requires additional passes of "while (Changed)"
iteration if to propagate data back from latter blocks to earlier blocks,
ordered according to depth_fist.
OR logic, used for ::May, converge to stable state faster then AND logic
use for ::Must.
Though the better solution is to switch to some some form of queue, but
having that this one is good enough, I will consider to do that later.
We can switch ::Must to OR logic if we calculate "may be dead" instead
of direct "must be alive" and then convert values to match existing
interface.
Additionally it fixes correctness in "@cycle" test.
Reviewed By: kstoimenov, fmayer
Differential Revision: https://reviews.llvm.org/D134796
01f3e2d619