[ScopInfo] Expect the OriginalBaseAddr when looking at underlying instructions [NFC]

During SCoP construction we sometimes inspect the underlying IR by looking at
the base address of a MemoryAccess. In such cases, we always want the original
base address. Make this clear by calling getOriginalBaseAddr().

This is a non-functional change as getBaseAddr maps to getOriginalBaseAddr
at the moment.

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

llvm-svn: 294576