[AMDGPU] gfx1010 Avoid SMEM WAR hazard for some s_waitcnt values

Summary:
Avoid introducing hazard mitigation when lgkmcnt is reduced to 0.
Clarify code comments to explain assumptions made for this hazard
mitigation.  Expand and correct test cases to cover variants of
s_waitcnt.

Reviewers: nhaehnle, rampitec

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62058

llvm-svn: 361124