IR: Add MDExpression::ExprOperand

Port `DIExpression::Operand` over to `MDExpression::ExprOperand`.  The
logic is needed directly in `MDExpression` to support printing in
assembly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@229002 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed