[m-rev.] for post-commit review: fix a perf bug

[Zoltan Somogyi]

The story of how I found the need for this fix is in the log message.

This diff  just works around the performance problem for code 
that does not use solver types. It does so by avoiding calls to
the actual problem predicate, candidate_init_vars.

That predicate was introduced into what was then modes.m in 2004,
in commit 3bbfd4c7c98ee54de9ab7a0aa2ea6ba31d0e3d9d. It was
added by rafe, so I cannot ask him, but maybe someone else has
worked on it since. Does anyone know why it is written the way it is?
If someone does, maybe we could actually solve the problem,
not just work around it.

Zoltan.
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