llvm.dbg.region.end() intrinsic is not required to be in _last_ basic block in...

llvm.dbg.region.end() intrinsic is not required to be in _last_ basic block in a function.  If that happens then any basic block that follows (lexically) the block with regin.end will not have scope info available.  LexicalScopeStack relies on processing basic block in CFG order, but this processing order is not guaranteed. Things get complicated when the optimizer gets a chance to optimizer IR with dbg intrinsics. 
Apply defensive patch to preserve at least one lexical scope till the end of function.

llvm-svn: 73282