[Verifier] Allow TBAA metadata on atomicrmw and atomiccmpxchg

This used to be allowed before r289402 by default (before r289402 you
could have TBAA metadata on any instruction), and while I'm not sure
that it helps, it does sound reasonable enough to not fail the verifier
and we have out-of-tree users who use this.

llvm-svn: 289872