LoopVectorize.cpp: Appease MSC16.

Excuse me, I hope msc16 builders would be fine till its end day.
Introduce nullptr then. ;)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199001 91177308-0d34-0410-b5e6-96231b3b80d8