bb-reorder crashes on potential-fall-through computed jumps

[Alexandre Oliva]

gcc.c-torture/compile/950613-1.c fails on SH with -m4 -O2.  The
problem is that the subsequent basic block is indeed one of the
potential edges, so the jump is considered as a potential
fall-through, so we end up crashing.

Here's a patch that arranges for us not to abort() on non-conditional
and conditional computed jumps (the latter was needed by another port
I have worked on, so I'm taking the opportunity to contribute the
code).

Ok to install?