[SimplifyCFG] Try to merge edge block when threading (PR55765)

When threading, we always create a new block for the threaded edge
(even if the edge is not critical), which will later get folded back
into the predecessor if possible. Depending on precise processing
order, this separate block may break the detection of trivial
cycles in the threading code, which normally avoids infinite
threading of loops. Explicitly merge the created edge block into
the predecessor to avoid this.

Fixes https://github.com/llvm/llvm-project/issues/55765.

Differential Revision: https://reviews.llvm.org/D127216