[IndVars] Pass the right loop to isLoopInvariantPredicate

[IndVars] Pass the right loop to isLoopInvariantPredicate

The loop on IVOperand's incoming values assumes IVOperand to be an
induction variable on the loop over which `S Pred X` is invariant;
otherwise loop invariant incoming values to IVOperand are not guaranteed
to dominate the comparision.

This fixes PR26973.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@263827 91177308-0d34-0410-b5e6-96231b3b80d8