LoopVectorize: Remove unnecessary RAUW

Remove an unnecessary `MDNode::replaceAllUsesWith()`.  In the preceding
line, `TheLoop->setLoopID()` visits all backedges and sets the new loop
ID.  This sufficiently updates the loop metadata.

Metadata RAUW is going away as part of PR21532.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223210 91177308-0d34-0410-b5e6-96231b3b80d8