Fix a secondary bug I introduced while fixing the first part of PR14478.

The first half of fixing this bug was actually in r170328, but was
entirely coincidental. It did however get me to realize the nature of
the bug, and adapt the test case to test more interesting behavior. In
turn, that uncovered the rest of the bug which I've fixed here.

This should fix two new asserts that showed up in the vectorize nightly
tester.

llvm-svn: 170333