[LICM] Avoind store sinking if no preheader is available

Load instructions are inserted into loop preheaders when sinking stores
and later removed if not used by the SSA updater. Avoid sinking if the
loop has no preheader and avoid crashes. This fixes one more side effect
of not handling indirectbr instructions properly on LoopSimplify.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223119 91177308-0d34-0410-b5e6-96231b3b80d8