The patchpoint lowering logic would crash with live constants equal to
the tombstone or empty keys of a DenseMap<int64_t, T>.  This patch
fixes the issue (and adds a tests case).



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221214 91177308-0d34-0410-b5e6-96231b3b80d8