[SLPVectorizer] Failure to beneficially vectorize 'copyable' elements in integer binary ops.

    Patch tries to improve vectorization of the following code:

    void add1(int * __restrict dst, const int * __restrict src) {
      *dst++ = *src++;
      *dst++ = *src++ + 1;
      *dst++ = *src++ + 2;
      *dst++ = *src++ + 3;
    }
    Allows to vectorize even if the very first operation is not a binary add, but just a load.

    Fixed PR34619 and other issues related to previous commit.

    Reviewers: spatel, mzolotukhin, mkuper, hfinkel, RKSimon, filcab, ABataev

    Reviewed By: ABataev, RKSimon

    Subscribers: llvm-commits, RKSimon

    Differential Revision: https://reviews.llvm.org/D28907

llvm-svn: 317618