[X86] Combine reduce(add (mul x, y)) to VNNI instruction.

For below C code, we can use VNNI to combine the mul and add operation.
int usdot_prod_qi(unsigned char *restrict a, char *restrict b, int c,
                  int n) {
  int i;
  for (i = 0; i < 32; i++) {
    c += ((int)a[i] * (int)b[i]);
  }
  return c;
}
We didn't support the combine acoss basic block in this patch.

Differential Revision: https://reviews.llvm.org/D116039