[UnifyFunctionExitNodes] Fix Modified status for unreachable blocks

If a function had at most one return block, the pass would return false
regardless if an unified unreachable block was created.

This patch fixes that by refactoring runOnFunction into two separate
helper functions for handling the unreachable blocks respectively the
return blocks, as suggested by @bjope in a review comment.

This was caught using the check introduced by D80916.

Reviewed By: serge-sans-paille

Differential Revision: https://reviews.llvm.org/D85818