Fixup PHIs in LowerSwitch when a Leaf node is not emitted.

This commit fixes bug http://llvm.org/bugs/show_bug.cgi?id=20103.

Thanks to Qwertyuiop for the report and the proposed fix.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212802 91177308-0d34-0410-b5e6-96231b3b80d8