While running pass #3661 SILFunctionTransform "Simplify CFG" on SILFunction "@TFC12ProcedureKit9Protector4readurfFxqdqd_".
The type in question is a generic abstraction to provide many-reads/single-write thread safety, see Protector in Sources/Support.swift
Also, this changeset was necessary to get round a Swift 3.0 (Xcode 8.0) release configuration compilation error (doesn't happen in debug configuration) - separate issue which you might be interested in too.
I'm looking into it. When I tried to reproduce with the ToT compiler, I'm running into a different issue. The compiler crashes in the devirtualizer. Seems like this new bug was introduced very recently.