Trac #14226 noted that the C-- CBE pass frequently fails to
common up semantically identical blocks due to the differences in local
register naming. These patches fixed this by making the pass consider
equality up to alpha-renaming.

However, the new logic failed to consider the possibility that local
register naming *may* matter across multiple blocks. This lead to the
regression #14754. I'll need to do a bit of thinking on a proper
solution to this but in the meantime I'm reverting all four patches.

This reverts commit a27056f9.
This reverts commit 6f990c54.
This reverts commit 9aa73892.
This reverts commit 7920a7d9.