[DependenceInfo] Use ScopArrayInfo to keep track of arrays [NFC]

When computing reduction dependences we first identify all ScopArrays which are
part of reductions and then only compute for these ScopArrays the more detailed
data dependences that allow us to identify reductions and optimize across them.
Instead of using the base pointer as identifier of a ScopArray, it is clearer
and more understandable to directly use the ScopArray as identifier. This change
implements such a switch.

This change removes unnecessary uses of MemoryAddress::getBaseAddr() in
preparation for https://reviews.llvm.org/D28518.

llvm-svn: 294567