SLPVectorizer: Fix stale for Value pointer array

We are slicing an array of Value pointers and process those slices in a loop.
The problem is that we might invalidate a later slice by vectorizing a former
slice.

Use a WeakVH to track the pointer. If the pointer is deleted or RAUW'ed we can
tell.

The test case will only fail when running with libgmalloc.

radar://15498655

llvm-svn: 195162