Add Note [Dead case binders in -O0]

After reverting Phab:D5358, Simon (Peyton Jones) asked for a Note
summarising why we want to keep the dead case binder check in `cgCase`.

Summary from mail conversation:

* Phab:D5324 means that we no longer /recompute/ dead-ness of case-binders in
  STG-land

* But TidyPgm preserves dead-ness info (see CoreTidy.tidyIdBndr)

* And so we can take advantage of it to avoid a redundant load. This load
  would be eliminated by CmmSink, but that only happens with -O

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