pp_leavewhen(): skip POPWHEN()

pp_leavewhen(): skip POPWHEN()

pp_leavewhen at some point will do an unconditional dounwind()
to pop the WHEN, BLOCK, and any other intermediate contexts, till it
returns to the enclosing FOR or GIVEN context. Thus there is no need to do
an initial POPBLOCK()/POPWHEN(), as this will be done by dounwind.
We just need to grab a couple of values from the top (GIVEN) context
to go arg stack processing correctly.