AMDGPU: IMPLICIT_DEFs and DBG_VALUEs do not contribute to wait states

AMDGPU: IMPLICIT_DEFs and DBG_VALUEs do not contribute to wait states

Summary:
This fixes a bug that was exposed on gfx9 in various
GL45-CTS.shaders.loops.*_iterations.select_iteration_count_fragment tests,
e.g. GL45-CTS.shaders.loops.do_while_uniform_iterations.select_iteration_count_fragment

Reviewers: arsenm

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

Differential Revision: https://reviews.llvm.org/D36193

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@312337 91177308-0d34-0410-b5e6-96231b3b80d8