Fixed consecutive memory access detection in Loop Vectorizer.

It did not handle correctly cases without GEP.

The following loop wasn't vectorized:

for (int i=0; i<len; i++)
  *to++ = *from++;

I use getPtrStride() to find Stride for memory access and return 0 is the Stride is not 1 or -1.

Differential revision: http://reviews.llvm.org/D20789

llvm-svn: 273257