Merge the unused header file for LoopVectorizer into the source file.

Merge the unused header file for LoopVectorizer into the source file.
This makes the loop vectorizer match the pattern followed by roughly all
other passses. =]

Notably, this header file was braken in several regards: it contained
a using namespace directive, global #define's that aren't globaly
appropriate, and global constants defined directly in the header file.

As a side benefit, lots of the types in this file become internal, which
will cause the optimizer to chew on this pass more effectively.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171723 91177308-0d34-0410-b5e6-96231b3b80d8