Remove a bad cast in CloneModule()

A cast that was introduced in r209007 was accidentally left in after the changes made to GlobalAlias rules in r210062. This crashes if the aliasee is a now-leggal ConstantExpr.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224756 91177308-0d34-0410-b5e6-96231b3b80d8