AMDGPU: Fix crash when scheduling non-memory SMRD instructions

Summary: Fixes piglit spec/arb_shader_clock/execution/*

Reviewers: arsenm

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

Differential Revision: https://reviews.llvm.org/D32345

llvm-svn: 301191