[ScopBuilder] Build escaping dependencies separately.

Instructions that compute escaping values might be synthesizable and
therefore not contained in any ScopStmt. When buildAccessFunctions is
changed to only iterate over the instruction list of statement,
"free" instructions still need to be written. We do this after the
main MemoryAccesses have been created.

This can change the order in which MemoryAccesses are created, but has
otherwise no functional change.

llvm-svn: 314663