Use .data() instead of &x[0]

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205722 91177308-0d34-0410-b5e6-96231b3b80d8
Use std::swap

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205723 91177308-0d34-0410-b5e6-96231b3b80d8