BBVectorize: Call a DAG and DAG instead of a tree

Several functions and variable names used the term 'tree' to refer
to what is actually a DAG. Correcting this mistake will, hopefully,
prevent confusion in the future.

No functionality change intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175278 91177308-0d34-0410-b5e6-96231b3b80d8