AMDGPU: Fix layering issue

Move utility function that depends on codegen.
Fixes build with r324487 reapplied.

llvm-svn: 324746
diff --git a/llvm/lib/Target/AMDGPU/SIISelLowering.cpp b/llvm/lib/Target/AMDGPU/SIISelLowering.cpp
index 83fe7e3..ab88376 100644
--- a/llvm/lib/Target/AMDGPU/SIISelLowering.cpp
+++ b/llvm/lib/Target/AMDGPU/SIISelLowering.cpp
@@ -1095,7 +1095,7 @@
 bool SITargetLowering::isMemOpUniform(const SDNode *N) const {
   const MemSDNode *MemNode = cast<MemSDNode>(N);
 
-  return AMDGPU::isUniformMMO(MemNode->getMemOperand());
+  return AMDGPUInstrInfo::isUniformMMO(MemNode->getMemOperand());
 }
 
 TargetLoweringBase::LegalizeTypeAction