When checking aliases between phi sources and V2, we know the sources are not themsel...

When checking aliases between phi sources and V2, we know the sources are not themselves phi nodes. However, V2 may be. Call aliasCheck with V2 first to potentially eliminate a std::swap call.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@84226 91177308-0d34-0410-b5e6-96231b3b80d8