Revert "[SimplifyCFG] Start redesigning `FoldTwoEntryPHINode()`."

Unfortunately, it seems we really do need to take the long route;
start from the "merge" block, find (all the) "dispatch" blocks,
and deal with each "dispatch" block separately, instead of simply
starting from each "dispatch" block like it would logically make sense,
otherwise we run into a number of other missing folds around
`switch` formation, missing sinking/hoisting and phase ordering.

This reverts commit 85628ce7.
This reverts commit c5fff909.
This reverts commit 34a98e10.
This reverts commit 1e353f09.