This fixes a regression introduced by r226816.
When replacing a splat shuffle node with a constant build_vector,
make sure that the new build_vector has a valid number of elements.
Thanks to Patrik Hagglund for reporting this problem and providing a
small reproducible.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@227002 91177308-0d34-0410-b5e6-96231b3b80d8
