[CodeGen] Fixed unreachable with -print-machineinstrs and custom pseudo source value

Summary:
Rev 327580 "[CodeGen] Use MIR syntax for MachineMemOperand printing"
broke -print-machineinstrs for us on AMDGPU, because we have custom
pseudo source values, and MIR serialization does not implement that.

This commit at least restores the functionality of -print-machineinstrs,
even if it does not properly implement the missing MIR serialization
functionality.

Differential Revision: https://reviews.llvm.org/D44871

Change-Id: I44961c0b90bf6d48c01484ed7a4e466fd300db66
llvm-svn: 328668