[PR25538]: Fix a failure caused by r253126.

[PR25538]: Fix a failure caused by r253126.

In r253126 we stopped to recompute LCSSA after loop unrolling in all
cases, except the unrolling is full and at least one of the loop exits
is outside the parent loop. In other cases the transformation should not
break LCSSA, but it turned out, that we also call SimplifyLoop on the
parent loop, which might break LCSSA by itself. This fix just triggers
LCSSA recomputation in this case as well.

I'm committing it without a test case for now, but I'll try to invent
one. It's a bit tricky because in an isolated test LoopSimplify would
be scheduled before LoopUnroll, and thus will change the test and hide
the problem.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@253253 91177308-0d34-0410-b5e6-96231b3b80d8