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Add a new pass that analyzes the module to find the minimal subtyping relation
that is necessary to maintain the validity and semantics of the program and
rewrites the types to use this minimal relation. Besides eliminating references
to otherwise-unused intermediate types, this optimization should unlock
significant additional optimizing power in other type optimizations that are
constrained by having to maintain supertype validity, since after this new
optimization there are fewer and more general supertypes.
The analysis works by visiting each expression and module element to collect the
subtypings that are required to maintain its validity, then, using that as a
starting point, iteratively adding new subtypings required by type definitions
and casts until reaching a fixed point.
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