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e25e192ef3
The `when` node now acts as a join point for patterns in the when
clause, with match/no-match completions. This is similar to how `or`
expressions work.
The result of this is that the `when` clause "controls" the body of the
`when`, which allows us to model barrier guards for multi-pattern when
clauses.
For this code
case x
when 1, 2
y
end
The old CFG was
x --> when --> 1 --no-match--> 2 ---no-match---> case
\ \ ^
\ \ |
\ --match----+ |
\ | |
\ | |
------match---------> y --+
The new CFG is
x --> 1 --no-match--> 2 --no-match--> [no-match] when --no-match--> case
\ \ ^
\ \ |
\ --match--> [match] when --match--> y -----+
\ /
\ /
-------match-----
i.e. all patterns flow to the `when` node, which is split based on
whether the pattern matched or not. The body of the when clause then has
a single predecessor `[match] when`, which acts as condition block that
controls `y`.