[Assignment Tracking] Elide a map copy in some cases

Restructure AssignmentTrackingLowering::join to avoid a map copy in the case
where BB has more than one pred.

We only need to perform a copy of a pred LiveOut if there's exactly one
already-visited pred (Result = PredLiveOut). With more than one pred the result
is built by calling Result = join(std::move(Result), PredLiveOut) for each
subsequent pred, where join parameters are const &. i.e. with more than 1 pred
we can avoid copying by referencing the first two pred LiveOuts in the first
join and then using a move + reference for the rest.

This reduces compile time for CTMark LTO-O3-g builds.

Reviewed By: jmorse

Differential Revision: https://reviews.llvm.org/D144732