[GISel][CallLowering] Make isIncomingArgumentHandler a pure virtual method

The default implementation of isIncomingArgumentHandler could lead
to generating incorrect code.
Make it a pure virtual method, so that targets know they have to
override it to produce correct code.

NFC

Differential Revision: https://reviews.llvm.org/D69187

llvm-svn: 375277
diff --git a/llvm/lib/Target/AMDGPU/AMDGPUCallLowering.cpp b/llvm/lib/Target/AMDGPU/AMDGPUCallLowering.cpp
index f3ad40f..58c44ac 100644
--- a/llvm/lib/Target/AMDGPU/AMDGPUCallLowering.cpp
+++ b/llvm/lib/Target/AMDGPU/AMDGPUCallLowering.cpp
@@ -37,6 +37,8 @@
 
   MachineInstrBuilder MIB;
 
+  bool isIncomingArgumentHandler() const override { return false; }
+
   Register getStackAddress(uint64_t Size, int64_t Offset,
                            MachinePointerInfo &MPO) override {
     llvm_unreachable("not implemented");