Loop Vectorizer: Handle pointer stores/loads in getWidestType()

In the loop vectorizer cost model, we used to ignore stores/loads of a pointer
type when computing the widest type within a loop. This meant that if we had
only stores/loads of pointers in a loop we would return a widest type of 8bits
(instead of 32 or 64 bit) and therefore a vector factor that was too big.

Now, if we see a consecutive store/load of pointers we use the size of a pointer
(from data layout).

This problem occured in SingleSource/Benchmarks/Shootout-C++/hash.cpp (reduced
test case is the first test in vector_ptr_load_store.ll).

radar://13139343