[SimplifyCFG] Preserve !annotation in FoldBranchToCommonDest.

When folding a branch to a common destination, preserve !annotation on
the created instruction, if the terminator of the BB that is going to be
removed has !annotation. This should ensure that !annotation is attached
to the instructions that 'replace' the original terminator.

Reviewed By: jdoerfert, lebedev.ri

Differential Revision: https://reviews.llvm.org/D93410