Avoid creating lots of mortals in B::walkoptree()

Avoid creating lots of mortals in B::walkoptree()

When calling out to the user-supplied method, re-use the same reference and
object where possible. Only create a new one if the user supplied method
modified the reference or object passed to it.

The previous implementation had a comment "Use the same opsv. Rely on methods
not to mess it up." but it was actually generating a new reference for every
call, and also a new object for every recursive call. So massive churn of
objects, and large accumulation of mortals on the temp stack.