[ScopBuilder] Specialize exit block handling. NFC.

Decouple handling of exit block PHIs and other MemoryAccesses. Exit PHIs
only need the PHI handling part of buildAccessFunctions but requires
code for skipping them in while creating other MemoryAcesses.

This change will make it easier to modify how statement MemoryAccesses
are created without considering the exit block special case.

llvm-svn: 314662