LoopVectorize: Invert case when we use a vector cmp value to query select cost

We generate a select with a vectorized condition argument when the condition is
NOT loop invariant. Not the other way around.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177098 91177308-0d34-0410-b5e6-96231b3b80d8