AMDGPU/SI: Use the hazard recognizer to break SMEM soft clauses

Summary:
Add support for detecting hazards in SMEM soft clauses, so that we only
break the clauses when necessary, either by adding s_nop or re-ordering
other alu instructions.

Reviewers: arsenm

Subscribers: arsenm, llvm-commits

Differential Revision: http://reviews.llvm.org/D18870

llvm-svn: 268260