cmd/compile: improve undo of poset

prove uses the poset datastructure in a DFS walk, and always undoes
it back to its pristine status. Before this CL, poset's undo of
a new node creation didn't fully deallocate the node, which means
that at the end of prove there was still some allocated memory pending.

This was not a problem until now because the posets used by prove
were discarded after each function, but it would prevent recycling
them between functions (as a followup CL does).

Change-Id: I1c1c99c03fe19ad765395a43958cb256f686765a
Reviewed-on: https://go-review.googlesource.com/112976


Run-TryBot: Giovanni Bajo <rasky@develer.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: David Chase <drchase@google.com>