[LoopUnrolling] Try harder to avoid rebuilding LCSSA when possible.

In r255133 (reapplied r253126) we started to avoid redundant
recomputation of LCSSA after loop-unrolling. This patch moves one step
further in this direction - now we can avoid it for much wider range of
loops, as we start to look at IR and try to figure out if the
transformation actually breaks LCSSA phis or makes it necessary to
insert new ones.

Differential Revision: http://reviews.llvm.org/D16838

llvm-svn: 259869