[Analysis] Fix merging TBAA tags with different final access types

There are cases when we have to merge TBAA access tags with the
same base access type, but different final access types. For
example, accesses to different members of the same structure may
be vectorized into a single load or store instruction. Since we
currently assume that the tags to merge always share the same
final access type, we incorrectly return a tag that describes an
access to one of the original final access types as the generic
tag. This patch fixes that by producing generic tags for the
common type and not the final access types of the original tags.

Resolves:
PR35225: Wrong tbaa metadata after load store vectorizer due to
recent change
https://bugs.llvm.org/show_bug.cgi?id=35225

Differential Revision: https://reviews.llvm.org/D39732

llvm-svn: 317682