[SLP] Fix PR40310: The reduction nodes should stay scalar.

Summary:
Sometimes the SLP vectorizer tries to vectorize the horizontal reduction
nodes during regular vectorization. This may happen inside of the loops,
when there are some vectorizable PHIs. Patch fixes this by checking if
the node is the reduction node and thus it must not be vectorized, it must
be gathered.

Reviewers: RKSimon, spatel, hfinkel, fedor.sergeev

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D56783

llvm-svn: 351349