I've removed the HasUncountableEarlyExit variable, since we can
already determine whether or not a loop has an early exit by seeing
if we found an uncountable exit.
I have also deleted the old UncountableExitingBlocks and
UncountableExitBlocks lists and replaced them with a single
uncountable edge. This means we don't need to worry about keeping the
list entries in sync and makes it clear which exiting block
corresponds to which exit block.
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