[AMDGPU] Allow any value in unused src0 field in v_nop

Summary:
The LLVM disassembler assumes that the unused src0 operand of v_nop is
zero. Other tools can put another value in that field, which is still
valid. This commit fixes the LLVM disassembler to recognize such an
encoding as v_nop, in the same way as we already do for s_getpc.

Differential Revision: https://reviews.llvm.org/D63724

Change-Id: Iaf0363eae26ff92fc4ebc716216476adbff37a6f
llvm-svn: 364208