Fix some bugs in step 2. Actually pass len_diff to expand_term and don't

Fix some bugs in step 2.  Actually pass len_diff to expand_term and don't
expand inactive spans (per the algorithm documentation).

In the interval tree addition, sanity check inputs so high is always higher
than low.

Add a sanity check to x86_bc_jmp_expand() to detect trying to expand an
already-expanded jump.

svn path=/branches/new-optimizer/; revision=1581