[ScopBuilder] Set domain to empty instead of NULL.

The domain generation used nullptr to mark the domain of an error block
as never-executed. Later, nullptr domains are recreated with a
zero-tuple domain that then mismatches with the expected domain the
error block within the loop.

Instead of using nullptr, assign an empty domain which preserves the
expected space. Remove empty domains during SCoP simplification.

Fixes llvm.org/PR38218.

llvm-svn: 338646