Make loop preheader insertion in LoopSimplify handle the case where the loop...

Make loop preheader insertion in LoopSimplify handle the case where the loop header is a landing pad correctly (by splitting the landingpad out of the loop header).  Make some adjustments to the rest of LoopSimplify to make it clear that the rest of LoopSimplify isn't making bad assumptions about the presence of landing pads.  PR11575.

llvm-svn: 146621