For blocks containing regexes, don't lower $_
For blocks containing regexes, don't lower $_

Since we need to look it up when doing `/foo/` in sink or boolean
contexts. Unfortunately, the current semantics of that rely on some
action at a distance (find the nearest defined `$_`, and update the `$/`
in that block); these probably want revisiting. I did attempt to try,
when we closure-clone a regex, to copy the appropriate `$_` into it;
this mostly works, but the case of `not /foo/` depends on it to skip
over the `not` frame, and there is another test which even further
relies on the current behavior. So for now, just don't optimize those
cases.

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