[CodeGen] Fix access sizes in new-format TBAA tags

The new format requires to specify both the type of the access
and its size. This patch fixes setting access sizes for TBAA tags
that denote accesses to structure members. This fix affects all
future TBAA metadata tests for the new format, so I guess we
don't need any special tests for this fix.

Differential Revision: https://reviews.llvm.org/D41452

llvm-svn: 321250