Prevent assert in CombinerGlobalAA with null values

DAGCombiner::isAlias can be called with SrcValue1 or SrcValue2 null, and we
can't use AA in this case (if we try, then the casting code in AA will assert).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190763 91177308-0d34-0410-b5e6-96231b3b80d8