GlobalOpt: Optimize in the face of insertvalue/extractvalue

GlobalOpt didn't know how to simulate InsertValueInst or
ExtractValueInst.  Optimizing these is pretty straightforward.

N.B. This came up when looking at clang's IRGen for MS ABI member
pointers; they are represented as aggregates.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215184 91177308-0d34-0410-b5e6-96231b3b80d8