[SLPVectorizer] Fix for PR25748: reduction vectorization after loop

unrolling.

The next code is not vectorized by the SLPVectorizer:
```
 int test(unsigned int *p) {
  int sum = 0;
  for (int i = 0; i < 8; i++)
    sum += p[i];
  return sum;
 }
```
During optimization this loop is fully unrolled and SLPVectorizer is
unable to vectorize it. Patch tries to fix this problem.

Differential Revision: https://reviews.llvm.org/D24796

llvm-svn: 283535