Use unique_ptr to handle ownership of TreePatterns in CodeGenDAGPatterns::PatternFragments

We might be able to use unique_ptr to handle ownership of the
TreePatternNodes too - looking into that next.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221928 91177308-0d34-0410-b5e6-96231b3b80d8