Fix gather-root.ll SLP vectorizer test to not expose UB.

The undefined behaviour (signed integer overflow) is not a regression
in itself as it was already there, but the test exposing it is a
regression compared to rc1, i.e. the lit tests no longer run ubsan-clean.

This commit fixes the test based on Matt's change in r279125 to not
expose the undefined behaviour.

llvm-svn: 279468