various DCE-related bugfixes (replaces #36888)#37147

Merged

vchuravy merged 6 commits intomasterfrom

Aug 22, 2020

Commits on Aug 21, 2020

Decline to remove phi nodes when this might cause forward references

yhls
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vchuravy
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Remove edges from dead blocks from phi nodes

yhls
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vchuravy
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Fix possible merge cycle bug in `cfg_simplify!`

yhls
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vchuravy
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Fix bug in `kill_edge!` which caused edges to be removed from the wrong phi nodes

yhls
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vchuravy
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Fix bug in which uses of SSA values processed in `just_fixup!` were not counted

yhls
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vchuravy
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Fix small bug in #36684

yhls
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vchuravy
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