[LoopNest] Use `getUniqueSuccessor()` instead when checking empty blocks

Blocks that contain only a single branch instruction to the next block can be skipped in analyzing the loop-nest structure.
This is currently done by `getSingleSuccessor()`.
However, the branch instruction might have multiple targets which happen to all be the same.
In this case, the block should still be considered as empty and skipped.

An example is `test/Transforms/LoopInterchange/update-condbranch-duplicate-successors.ll` (the LIT test for this patch is modified from it as well).

Reviewed By: Whitney

Differential Revision: https://reviews.llvm.org/D97286