swift234

SimplifyCFG: Fix a bug in the jump threading code

When jump threading a block we used to propagate phi values directly into the
threaded block - instead of leaving in a copy. Because of this the SSA updater
would propagate the value feeding the copy from the next iteration.
Now when jump threading the destination block into the edge we leave in the phi
(copy) such that the SSA updater picks up value of the right iteration.

rdar://20617338

Swift SVN r27722