AMDGPU/SI: Pass whether to use the SI scheduler via Target Attribute

Summary:
Currently the SI scheduler can be selected via command line option,
but it turned out it would be better if it was selectable via a Target Attribute.

This patch adds "si-scheduler" attribute to the backend.

Reviewers: tstellarAMD, echristo

Subscribers: echristo, arsenm

Differential Revision: http://reviews.llvm.org/D16192

llvm-svn: 258386
diff --git a/llvm/lib/Target/AMDGPU/AMDGPUTargetMachine.cpp b/llvm/lib/Target/AMDGPU/AMDGPUTargetMachine.cpp
index ab2b862..7da7fad 100644
--- a/llvm/lib/Target/AMDGPU/AMDGPUTargetMachine.cpp
+++ b/llvm/lib/Target/AMDGPU/AMDGPUTargetMachine.cpp
@@ -148,6 +148,8 @@
     const AMDGPUSubtarget &ST = *getAMDGPUTargetMachine().getSubtargetImpl();
     if (ST.getGeneration() <= AMDGPUSubtarget::NORTHERN_ISLANDS)
       return createR600MachineScheduler(C);
+    else if (ST.enableSIScheduler())
+      return createSIMachineScheduler(C);
     return nullptr;
   }