cmm/CBE: Fix comparison between blocks of different lengths

Previously CBE computed equality by taking the lists of middle nodes of
the blocks being compared and zipping them together. It would then map
over this list with the equality relation, and accumulate the result.

However, this is completely wrong: Consider what will happen when we
compare a block with no middle nodes with one with one or more. The
result of `zip` will be empty and consequently the pass may conclude
that the two are indeed equivalent (if their last nodes also match).
This is very bad and the cause of #14361.

The solution I chose was just to write out an explicit recursion, like I
distinctly recall considering doing when I first wrote this code.
Unfortunately I was feeling clever at the time.

Unfortunately this case was just rare enough not to be triggered by the
testsuite. I still need to find a testcase that doesn't have external
dependencies.

Test Plan: Need to find a more minimal testcase

Reviewers: austin, simonmar, michalt

Reviewed By: michalt

Subscribers: michalt, rwbarton, thomie, hvr

GHC Trac Issues: #14361

Differential Revision: https://phabricator.haskell.org/D4152