[SLP] Improve horizontal vectorization for non-power-of-2 number of

instructions.

If number of instructions in horizontal reduction list is not power of 2
then only PowerOf2Floor(NumberOfInstructions) last elements are actually
vectorized, other instructions remain scalar. Patch tries to vectorize
the remaining elements either.

Differential Revision: https://reviews.llvm.org/D28959

llvm-svn: 293042