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	Remove all vestiges of usage analysis
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This commit removes a large blob of usage-analysis-related code, almost
all of which was commented out.

Sadly, it doesn't look as if Keith is going to have enough time to polish it
up, and in any case the actual performance benefits (so far as we can measure
them) turned out to be pretty modest (a few percent).

So, with regret, I'm chopping it all out.  It's still there in the repository
if anyone wants go hack on it.  And Tobias Gedell at Chalmers is implementing
a different analysis, via External Core.