[mlir][llvm] Fix TBAA verfication crash

This commit fixes a crash of the TBAA verification that happened due to
accessing memory through invalid pointers. A DenseMap does not guarantee
that pointers to its elements remain valid after additional elements
are inserted.

A testcase that caused this crash had more than 100 TBAA metadata
operations and thus no test is added. Instead, there is now an assertion
that ensures that the graph class is used correctly.

Reviewed By: vzakhari

Differential Revision: https://reviews.llvm.org/D143653