In `manifestSp` the unwind info was before the relevant instruction, not
after.  I added some notes to establish semantics.  Also removes
redundant annotation in stg_catch_frame.

For `makeFixupBlocks` it looks like we were off by `wORD_SIZE dflags`.
I'm not sure why, but it lines up with `manifestSp`.  In fact it lines
up so well so that I can consolidate the Sp unwind logic in
`maybeAddUnwind`.  I detected the problems with `makeFixupBlocks` by
running T14779b after patching D4559.

Test Plan: added a new test

Reviewers: bgamari, scpmw, simonmar, erikd

Reviewed By: bgamari

Subscribers: thomie, carter

GHC Trac Issues: #14999

Differential Revision: https://phabricator.haskell.org/D4606