Don't recompute LCSSA after loop-unrolling when possible.

Summary:
Currently we always recompute LCSSA for outer loops after unrolling an
inner loop. That leads to compile time problem when we have big loop
nests, and we can solve it by avoiding unnecessary work. For instance,
if w eonly do partial unrolling, we don't break LCSSA, so we don't need
to rebuild it. Also, if all exits from the inner loop are inside the
enclosing loop, then complete unrolling won't break LCSSA either.

I replaced unconditional LCSSA recomputation with conditional recomputation +
unconditional assert and added several tests, which were failing when I
experimented with it.

Soon I plan to follow up with a similar patch for recalculation of dominators
tree.

Reviewers: hfinkel, dexonsmith, bogner, joker.eph, chandlerc

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D14526

llvm-svn: 253126