Reapply "[SLP] Initialize VectorizedValue when gathering"

The test case included in r279125 exposed existing undefined behavior in the
SLP vectorizer that it did not introduce. This patch reapplies the original
patch, but modifies the test case to avoid hitting the undefined behavior. This
allows us to close PR28330 while keeping the UBSan bot happy. The undefined
behavior the original test uncovered will be addressed in a follow-on patch.

Reference: https://llvm.org/bugs/show_bug.cgi?id=28330
llvm-svn: 279370