Add iterators for the ArrayInfo objects of the scop

This patch also changes the implementation of the ArrayInfoMap to a MapVector
which will ensure that iterating over the list of ArrayInfo objects gives
predictable results. The single loop that currently enumerates the ArrayInfo
objects only frees the individual objectes, hence a possibly changing
iteration order does not affect the outcome. The added robustness is for
future users of this interface.

llvm-svn: 236583