[ScopInfo] Use ScopArrayInfo instead of base address

When building alias groups, we sort different ScopArrays into unrelated groups.
Historically we identified arrays through their base pointer, as no
ScopArrayInfo class was yet available. This change changes the alias group
construction to reference arrays through their ScopArrayInfo object.

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

llvm-svn: 294649