various DCE-related bugfixes (replaces #36888)#37147
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Phi nodes are optimized away when there is only one predecessor, but this can cause problems in dead loops because forward references can be created, leading to issues with optimization passes that look at all code, dead or not. This fixes issue #29107 when DCE is turned on.
…ot counted This fixes #29253, which was caused by `simple_dce!` erroneously erasing SSA values that did not appear to be used, because these uses were only discovered in `just_fixup!` at the end of iterating over an `IncrementalCompact`.
PR #36684 changes `iterate(IncrementalCompact)` to return an extra index, but leaves its arguments unchanged. However, the PR decremented the index argument in a particular recursive call to `iterate`. This caused `iterate` not to recognise that it was done when `allow_cfg_transforms` was turned on.
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Replaces #36888 and addresses the review comment there.