Trivial do-while loop crashes Prepack

[NTillmann]

let x = __abstract("number", "(x)");
do { x++; } while (x < 3);

=>

Invariant Violation:
This is likely a bug in Prepack, not your code. Feel free to open an issue on GitHub.
    at invariant (/Users/nikolait/git/prepack/lib/invariant.js:24:15)
    at ResidualOperationSerializer._serializeLocalAssignment (/Users/nikolait/git/prepack/lib/serializer/ResidualOperationSerializer.js:633:28)
    at ResidualOperationSerializer.serializeStatement (/Users/nikolait/git/prepack/lib/serializer/ResidualOperationSerializer.js:138:26)
    at TemporalOperationEntry.serialize (/Users/nikolait/git/prepack/lib/utils/generator.js:155:26)
    at serializeFn (/Users/nikolait/git/prepack/lib/utils/generator.js:852:46)
    at Generator.serialize (/Users/nikolait/git/prepack/lib/utils/generator.js:861:7)
    at _withGeneratorScope (/Users/nikolait/git/prepack/lib/serializer/ResidualHeapSerializer.js:1946:141)
    at ResidualHeapSerializer._withGeneratorScope (/Users/nikolait/git/prepack/lib/serializer/ResidualHeapSerializer.js:1907:5)
    at Object.serializeGenerator (/Users/nikolait/git/prepack/lib/serializer/ResidualHeapSerializer.js:1946:64)
    at ResidualOperationSerializer._serializeDoWhile (/Users/nikolait/git/prepack/lib/serializer/ResidualOperationSerializer.js:472:30)

The issue is that the phiNode is undefined.