[SimplifyCFG] Keep !dgb metadata of moved instruction, if they match.

Currently SimplifyCFG drops the debug locations of 'bonus' instructions.
Such instructions are moved before the first branch. The reason for the
current behavior is that this could lead to surprising debug stepping,
if the block that's folded is dead.

In case the first branch and the instructions to be folded have the same
debug location, this shouldn't be an issue and we can keep the debug
location.

Reviewed By: vsk

Differential Revision: https://reviews.llvm.org/D93662