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.

llvm-svn: 89484