AMDGPU/MC: Allow disassembling without symbol info

Summary:
We would like the UMR debugging tool[0] to be able to provide
disassembly for currently live waves based on plain memory
dumps, and we want to leverage the LLVM disassembler for this.

This mostly works, except that UMR clearly can't provide real
symbol info, so it wants to set DisInfo == nullptr.

[0] https://cgit.freedesktop.org/amd/umr/

Reviewers: arsenm, rampitec, artem.tamazov, dp

Subscribers: kzhuravl, wdng, yaxunl, dstuttard, tpr, t-tye, llvm-commits

Differential Revision: https://reviews.llvm.org/D45477

Change-Id: Ibb2c5af2e66f2e100b4702fd81308e1932bc4ee6
llvm-svn: 329715