-/* Propagate flow information through back edges and thus enable PRE's
- moving loop invariant calculations out of loops.
-
- Originally this tended to create worse overall code, but several
- improvements during the development of PRE seem to have made following
- back edges generally a win.
-
- Note much of the loop invariant code motion done here would normally
- be done by loop.c, which has more heuristics for when to move invariants
- out of loops. At some point we might need to move some of those
- heuristics into gcse.c. */
-