Revert "[DebugInfo] Generate debug information for labels. (Fix PR37395)"

Build failed in
http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux-autoconf/builds/27258

In lib/CodeGen/LiveDebugVariables.cpp:589, it uses std::prev(MBBI) to
get DebugValue's SlotIndex. however, the previous instruction may be
also a debug instruction.

llvm-svn: 338992