[IR][OpaquePointers] Properly print cmpxchg with pointer operands.

With opaque pointers address of pointer variable and its value have
same type (`ptr`). As a result, cmpxchg is printed without values
types in LLVM assembly and cannot be read back. Add AtomicCmpXchg
to the list of instructions which always have operand types printed.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D129276