CodeFolding: Fix up old EH when we fold away an If

[kripken]

Context: The pass does (among other things) this:

(if
  condition
  X
  X
)

=>

(block
  (drop
    condition
  )
  X ;; deduplicated
)

After that the condition is now nested in a block, so we may need EH fixups
if it contains a pop.

[aheejin]

Thanks!