[RISCV][GISel] Minor refactoring of RISCVCallReturnHandler and RISCVIncomingValueHandler to match other targets (llvm#69757)

Forward assignValueToReg to the base class to make the copy. Add
markPhysRegUsed to contain the differences between call handling and
argument handling. Introduce RISCVFormalArgHandler.

This structure matches how AArch64, AMDGPU, and X86 are structured.

I've also added `MIRBuilder.getMRI()->addLiveIn(PhysReg);` to match the
other targets.

Author: topperc

llvm/lib/Target/RISCV/GISel/RISCVCallLowering.cpp

@@ -164,28 +164,39 @@ struct RISCVIncomingValueHandler : public CallLowering::IncomingValueHandler {
 
   void assignValueToReg(Register ValVReg, Register PhysReg,
                         CCValAssign VA) override {
-    // Copy argument received in physical register to desired VReg.
-    MIRBuilder.getMBB().addLiveIn(PhysReg);
-    MIRBuilder.buildCopy(ValVReg, PhysReg);
+    markPhysRegUsed(PhysReg);
+    IncomingValueHandler::assignValueToReg(ValVReg, PhysReg, VA);
   }
 
+  /// How the physical register gets marked varies between formal
+  /// parameters (it's a basic-block live-in), and a call instruction
+  /// (it's an implicit-def of the BL).
+  virtual void markPhysRegUsed(MCRegister PhysReg) = 0;
+
 private:
   const RISCVSubtarget &Subtarget;
 };
 
+struct RISCVFormalArgHandler : public RISCVIncomingValueHandler {
+  RISCVFormalArgHandler(MachineIRBuilder &B, MachineRegisterInfo &MRI)
+      : RISCVIncomingValueHandler(B, MRI) {}
+
+  void markPhysRegUsed(MCRegister PhysReg) override {
+    MIRBuilder.getMRI()->addLiveIn(PhysReg);
+    MIRBuilder.getMBB().addLiveIn(PhysReg);
+  }
+};
+
 struct RISCVCallReturnHandler : public RISCVIncomingValueHandler {
   RISCVCallReturnHandler(MachineIRBuilder &B, MachineRegisterInfo &MRI,
                          MachineInstrBuilder &MIB)
       : RISCVIncomingValueHandler(B, MRI), MIB(MIB) {}
 
-  MachineInstrBuilder MIB;
-
-  void assignValueToReg(Register ValVReg, Register PhysReg,
-                        CCValAssign VA) override {
-    // Copy argument received in physical register to desired VReg.
+  void markPhysRegUsed(MCRegister PhysReg) override {
     MIB.addDef(PhysReg, RegState::Implicit);
-    MIRBuilder.buildCopy(ValVReg, PhysReg);
   }
+
+  MachineInstrBuilder MIB;
 };
 
 } // namespace
@@ -312,7 +323,7 @@ bool RISCVCallLowering::lowerFormalArguments(MachineIRBuilder &MIRBuilder,
   RISCVIncomingValueAssigner Assigner(
       CC == CallingConv::Fast ? RISCV::CC_RISCV_FastCC : RISCV::CC_RISCV,
       /*IsRet=*/false);
-  RISCVIncomingValueHandler Handler(MIRBuilder, MF.getRegInfo());
+  RISCVFormalArgHandler Handler(MIRBuilder, MF.getRegInfo());
 
   return determineAndHandleAssignments(Handler, Assigner, SplitArgInfos,
                                        MIRBuilder, CC, F.isVarArg());