Assert that dominates is not given a multiple edge. Finding out if we have

Assert that dominates is not given a multiple edge. Finding out if we have
multiple edges between two blocks is linear. If the caller is iterating all
edges leaving a BB that would be a square time algorithm. It is more efficient
to have the callers handle that case.

Currently the only callers are:
* GVN: already avoids the multiple edge case.
* Verifier: could only hit this assert when looking at an invalid invoke. Since
it already rejects the invoke, just avoid computing the dominance for it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@162113 91177308-0d34-0410-b5e6-96231b3b80d8