Use zip_longest for iterator range comparisons. NFC.

Use zip_longest in two locations that compare iterator ranges.
zip_longest allows the iteration using a range-based for-loop and to be
symmetric over both ranges instead of prioritizing one over the other.
In that latter case code have to handle the case that the first is
longer than the second, the second is longer than the first, and both
are of the same length, which must partially be checked after the loop.

With zip_longest, this becomes an element comparison within the loop
like the comparison of the elements themselves. The symmetry makes it
clearer that neither the first and second iterators are handled
differently. The iterators are not event used directly anymore, just
the ranges.

Differential Revision: https://reviews.llvm.org/D55468

llvm-svn: 348762