[m-rev.] diff: speed up equiv_types_hlds.m

[Zoltan Somogyi]

I had a look at how the compiler is handling Dirk's problematic module.
Almost all the time is spent in mode checking, either during the initial
mode analysis or reruns of it during e.g. simplification, with only two other
passes taking significant amounts of time on their own: equivalence
type expansion and quantification. This diff speeds up the former,
taking it from this:

% Fully expanding equivalence types... done.
[User time: +1.240s, 9.410s, Real time: +1.241s, 9.500s, D Stack: 4.109k, ND Stack: 0.078k, C Stack: 10.415k,

to this:

% Fully expanding equivalence types... done.
[User time: +0.200s, 8.420s, Real time: +0.206s, 8.506s, D Stack: 4.109k, ND Stack: 0.078k, C Stack: 10.415k,

I also have an idea for how to speed up mode checking, but that
is a more involved change.

I am committing this diff on the master branch. Julien, you may want
to apply it to the release branch as well. BTW, I did *not* find any
significant difference between the speed of the 14.01 and the current
compilers on Dirk's test case, though the current compiler did report
a bug in one of his mode declarations.

Zoltan.
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