TBAA: use the same format for scalar TBAA and struct-path aware TBAA.

An updated version of r191586 with bug fix.

Struct-path aware TBAA generates tags to specify the access path,
while scalar TBAA only generates tags to scalar types.

We should not generate a TBAA tag with null being the first field. When
a TBAA type node is null, the tag should be null too. Make sure we
don't decorate an instruction with a null TBAA tag.

Added a testing case for the bug reported by Richard with -relaxed-aliasing
and -fsanitizer=thread.