UnTODO test, we now support non-jittable subtrees.
Export pj_binop_sassign.
Fix ANY/SV stringification.
Handle trees with multiple last ops (depending on a condition).

The code for emitting subtrees assumed that control flow always
exited through the root node of the subtree.  Conditional ops
don't follow this rule, and have multiple exit points from the
subtree.
JIT scalar assignment.

Note that the code only handles the cases where the left-hand side
of the assignment returns a Perl scalar; this should be the case for
most "normal" code.  The implementation for the cases where the LHS
is not a scalar is being worked on in a separate branch.