Fix bug in parser optimizer whereby local bindings for the buffer

Fix bug in parser optimizer whereby local bindings for the buffer
pointer would be eliminated due to ostensible lack of reference, even
if the user code contained references to them, because the reference
counter won't descend into the user code.  Also, reintroduce the new
rule, which was previously removed, for turning redundant bindings of
the buffer pointer into aliases.  Cases affected independently by each
optimization should now be covered correctly & safely.