[NFCI-ish][SimplifyCFGPass] Rework and generalize `ret` block tail-merging

This changes the approach taken to tail-merge the blocks
to always create a new block instead of trying to reuse some block,
and generalizes it to support dealing not with just the `ret` in the future.

This effectively lifts the CallBr restriction, although this isn't really intentional.
That is the only non-NFC change here, i'm not sure if it's reasonable/feasible to temporarily retain it.

Other restrictions of the transform remain.

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D104598