Fix union simplification performance regression#12519
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jhance
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Apr 4, 2022
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#11962 can generate large unions with many
Instancetypes withlast_known_valueset. This caused our union simplification algorithmto be extremely slow, as it hit an O(n**2) code path.
We already had a fast code path for unions of regular literal types. This
generalizes it for unions containing
Instancetypes with last knownvalues (which behave similarly to literals in a literal type context).
Also fix a union simplification bug that I encountered while writing tests
for this change.
Work on #12408.