[SwitchLowering] Handle destinations on multiple phi instructions

Follow up from r222926. Also handle multiple destinations from merged
cases on multiple and subsequent phi instructions.

rdar://problem/19106978

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223135 91177308-0d34-0410-b5e6-96231b3b80d8