[DebugInfo] Fix bug in LiveDebugVariables.
In lib/CodeGen/LiveDebugVariables.cpp, it uses std::prev(MBBI) to
get DebugValue's SlotIndex. However, the previous instruction may be
also a debug instruction. It could not use a debug instruction to query
SlotIndex in mi2iMap.
Scan all debug instructions and use the first debug instruction to query
SlotIndex for following debug instructions. Only handle DBG_VALUE in
handleDebugValue().
Differential Revision: https://reviews.llvm.org/D50621
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@341446
91177308-0d34-0410-b5e6-
96231b3b80d8