Backport mark loop rewrite #26
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It's possible for PiNodes to effectively imply statically the condition
of a Core.ifelse. For example:
```julia
23 ─ %60 = Core.ifelse(%47, false, true)::Bool
│ %61 = Core.ifelse(%47, %58, false)::Union{Missing, Bool}
25 ─ goto #27 if not %60
26 ─ %65 = π (%61, Bool)
└─── ...
```
In basic block #26, the PiNode gives us enough information to conclude
that `%47 === false` if control flow reaches that point. The previous
code incorrectly assumed that this kind of pruning would only be done
for PhiNodes.
Resolves JuliaLang#50276
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Backport of an early version of the mark-loop rewrite.
This PR has a known regression that was addressed on later JuliaLang PRs, so it shouldn't be used for performance analysis.
Opening it so that we have an additional checkpoint while we backport the multithreaded GC.
X-ref: JuliaLang#47292.