[SLP] Fix for PR31690: Allow using of extra values in horizontal reductions.

Currently LLVM supports vectorization of horizontal reduction
instructions with initial value set to 0. Patch supports vectorization
of reduction with non-zero initial values. Also it supports a
vectorization of instructions with some extra arguments, like:

float f(float x[], int a, int b) {
  float p = a % b;
  p += x[0] + 3;
  for (int i = 1; i < 32; i++)
    p += x[i];
  return p;
}

Patch allows vectorization of this kind of horizontal reductions.

Differential Revision: https://reviews.llvm.org/D28961

llvm-svn: 293994