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The previous algorithm scaled poorly when there was a large number of
blocks and edges.
The algorithm links together block chains which have edges between them
in the CFG. The new algorithm uses a union find data structure in order
to efficiently merge together blocks and calculate which block chain
each block id belonds to.
I copied the UnionFind data structure which already existed in Cabal
into the GHC library rathert than reimplement it myself.
This change results in a very significant reduction in allocations when
compiling the mmark package.
Ticket: #19471
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