Use the LiveVariables information to efficiently get local kills. This

removes the largest scaling problem in the test cases from PR13225 when
ASan is switched to insert basic blocks in the natural CFG order.

It may also solve some scaling problems for more normal code with large
numbers of basic blocks and variables.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@160194 91177308-0d34-0410-b5e6-96231b3b80d8