[LV] Avoid creating empty reduction entries (NFC)

[LV] Avoid creating empty reduction entries (NFC)

This patch prevents us from unintentionally creating entries in the reductions
map for PHIs that are not actually reductions. This is currently not an issue
since we bail out if we encounter PHIs other than inductions or reductions.
However the behavior could become problematic as we add support for additional
recurrence types.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@256930 91177308-0d34-0410-b5e6-96231b3b80d8