AMDGPU: Refactor argument lowering

Split into smaller functions and prepare for handling
non-entry functions.

llvm-svn: 299998
diff --git a/llvm/lib/Target/AMDGPU/R600ISelLowering.cpp b/llvm/lib/Target/AMDGPU/R600ISelLowering.cpp
index 3db74d4..3590a9b 100644
--- a/llvm/lib/Target/AMDGPU/R600ISelLowering.cpp
+++ b/llvm/lib/Target/AMDGPU/R600ISelLowering.cpp
@@ -1547,7 +1547,7 @@
   SmallVector<ISD::InputArg, 8> LocalIns;
 
   if (AMDGPU::isShader(CallConv)) {
-    AnalyzeFormalArguments(CCInfo, Ins);
+    CCInfo.AnalyzeFormalArguments(Ins, CCAssignFnForCall(CallConv, isVarArg));
   } else {
     analyzeFormalArgumentsCompute(CCInfo, Ins);
   }