[AliasSetTracker] Don't drop AA MD so eagerly

Summary:
When we have patterns like
loop:
    %la = load %ptr, !tbaa
    %lba = load %ptr, !tbaa !noalias

AliasSetTracker would previously think that the two types of annotation for
the pointer conflict, dropping both for the purpose of determining alias sets.
That is clearly way too conservative, as the tbaa is still valid whether or
not one of the memory accesses has additional AA metadata. We could go
one step further and attempt to properly merge the AA metadata,
but it's not clear that that would be worth it since that may introduce
additional MD nodes, which may be undesirable since this is merely an
Analysis.

Reviewers: hfinkel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D32139

llvm-svn: 306727