LoopVectorize: Use static function instead of DebugLocSetter class

I used the class to safely reset the state of the builder's debug location.  I
think I have caught all places where we need to set the debug location to a new
one. Therefore, we can replace the class by a function that just sets the debug
location.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185165 91177308-0d34-0410-b5e6-96231b3b80d8