[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.

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

Subscribers: llvm-commits, RKSimon

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

llvm-svn: 313348