SelectionDAG: If llvm.donothing has a landingpad, we should clear
CurrentCallSite to avoid an assertion failure:
assert(MMI.getCurrentCallSite() == 0 && "Overlapping call sites!");

rdar://problem/13228754


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@176154 91177308-0d34-0410-b5e6-96231b3b80d8