Before only u0 was special cased in handling a dereference;
now all zero bit types are handled the same way. Dereferencing
a pointer to a zero bit type always gives a comptime-known
result.

This previously had been failing on master branch but went unnoticed
because until 846f72b57cb5 the CI server was mistakenly not actually
running the single-threaded tests with --single-threaded.

This fixes the standard library tests with --single-threaded
--release-fast.