IR: Properly canonicalize PointerType in ConstantExpr GEPs

No additional test was needed, Other/constant-fold-gep.ll detects this
just fine.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194221 91177308-0d34-0410-b5e6-96231b3b80d8