[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 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: 318193