[GlobalISel][CallLowering] Rename isArgumentHandler() -> isIncomingArgumentHandler()

Previous name and comment incorrectly implied it was just for formal arg handlers,
which is not true.

llvm-svn: 367945
diff --git a/llvm/lib/Target/AMDGPU/AMDGPUCallLowering.cpp b/llvm/lib/Target/AMDGPU/AMDGPUCallLowering.cpp
index 905443f..6b194b1 100644
--- a/llvm/lib/Target/AMDGPU/AMDGPUCallLowering.cpp
+++ b/llvm/lib/Target/AMDGPU/AMDGPUCallLowering.cpp
@@ -128,7 +128,7 @@
   virtual void markPhysRegUsed(unsigned PhysReg) = 0;
 
   // FIXME: What is the point of this being a callback?
-  bool isArgumentHandler() const override { return true; }
+  bool isIncomingArgumentHandler() const override { return true; }
 };
 
 struct FormalArgHandler : public IncomingArgHandler {