BlockGenerator: Register outside users of scalars directly

Instead of checking at code generation time for each ScopStmt if a scalar has
external uses, we just iterate over the ScopArrayInfo descriptions we have and
check each of these for possible external uses.

Besides being somehow clearer, this approach has the benefit that we will always
create valid LLVM-IR even in case we disable the code generation of ScopStmt
bodies e.g. for testing purposes.

llvm-svn: 250608