[IR] Add classof for GetElementPtrConstantExpr, CompareConstantExpr,...

[IR] Add classof for GetElementPtrConstantExpr, CompareConstantExpr, InsertValueConstantExpr, and ExtractValueConstantExpr. All but CompareConstantExpr were being used in casts that were erroneously using ConstantExpr::classof due to inheritance. While there use cast<CompareConstantExpr> to simplify code slightly.

I believe in one place we were always casting to ExtractValueConstantExpr when we were trying to choose between ExtractValueConstantExpr and InsertValueConstantExpr because of this. But since they have identical layouts this didn't cause any observable problems.

llvm-svn: 255624