Cannot get Bit test RTL to cooperate with Combine.

[Joern Rennecke]

Quoting Andrew Hutchinson <andrewhutchinson@cox.net>:

> Why doesn't combine try matching "unsimplified" expressions when it fails?
>
> This would at least permit creating patterns based on explicit format
> of  input RTL without the added vagaries of simplification

Actually, that was my first attempt to approach the issue, but the
trouble with this is that it is ill-defined what the simplifications
are you want to loose versus what necessary canonicalisations are.

Ultimatively, the search space is infinite, and the patterns that combine
creates are shaped by the preconceptions of the programmers who hack it
on what patterns are good to match.  So it makes sense to try multiple
patterns for a single combination and test the cost, but we couldn't
afford an exhaustive search of all combinatorial possibilities.