Make Loop::getLoopLatch() work on loops which don't have preheaders, as

Make Loop::getLoopLatch() work on loops which don't have preheaders, as
it may be used in contexts where preheader insertion may have failed due
to an indirectbr.

Make LoopSimplify's LoopSimplify::SeparateNestedLoop properly fail in
the case that it would require splitting an indirectbr edge.

These fix PR5502.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@89484 91177308-0d34-0410-b5e6-96231b3b80d8