MFC r338689:

Pull in r335365 from upstream llvm trunk (by Krzysztof Parzyszek):

Initialize LiveRegs once in BranchFolder::mergeCommonTails

This should fix '(TRI && "LivePhysRegs is not initialized."' assertions
when building the lang/qt5-qml port in certain configurations.

Reported by: Piotr Kubaj <pkubaj@anongoth.pl>
PR: 231355