Fix some Type::Null stuff, LbGbl guard relaxation

Summary: - We were being sloppy with Type::Null in isKnownDataType and toDataType,
  mostly for historical reasons. I cleaned that up and nothing seemed to break.
- IsType<Null> was just checking for KindOfNull. I fixed it to check for <=
  KindOfNull to also get KindOfUninit.
- LdGbl is roughly the same as LdLoc but we weren't tracing through it in guard
  relaxation.

The new test is fixed by having either the second or the third item, since it
was a combination of both causing issues.

Reviewed By: @ottoni

Differential Revision: D1401528