This recognises barriers of the form
STRINGS = ["foo", "bar"]
case foo
when "some string literal"
foo
when *["other", "strings"]
foo
when *STRINGS
foo
end
where the reads of `foo` inside each `when` are guarded by the comparison
of `foo` with the string literals.
We don't yet recognise this construct:
case foo
when "foo", "bar"
foo
end
This is due to a limitation in the shared barrier guard logic.
