[Target] Remove unused forward declarations (NFC)

Author: kazutakahirata

llvm/lib/Target/AMDGPU/AMDGPU.h

@@ -17,7 +17,6 @@
 namespace llvm {
 
 class AMDGPUTargetMachine;
-class GCNTargetMachine;
 class TargetMachine;
 
 // GlobalISel passes

llvm/lib/Target/AMDGPU/AMDGPUHSAMetadataStreamer.h

@@ -30,7 +30,6 @@ class MDNode;
 class Module;
 struct SIProgramInfo;
 class Type;
-class GCNSubtarget;
 
 namespace AMDGPU {
 

llvm/lib/Target/AMDGPU/AMDGPUMachineFunction.h

@@ -20,7 +20,6 @@
 namespace llvm {
 
 class AMDGPUSubtarget;
-class GCNSubtarget;
 
 class AMDGPUMachineFunction : public MachineFunctionInfo {
   /// A map to keep track of local memory objects and their offsets within the

llvm/lib/Target/AMDGPU/Utils/AMDGPUMemoryUtils.h

@@ -14,12 +14,10 @@ namespace llvm {
 struct Align;
 class AAResults;
 class DataLayout;
-class Function;
 class GlobalVariable;
 class LoadInst;
 class MemoryDef;
 class MemorySSA;
-class Module;
 class Value;
 
 namespace AMDGPU {

llvm/lib/Target/Hexagon/HexagonTargetMachine.h

@@ -21,8 +21,6 @@
 
 namespace llvm {
 
-class Module;
-
 class HexagonTargetMachine : public LLVMTargetMachine {
   std::unique_ptr<TargetLoweringObjectFile> TLOF;
   mutable StringMap<std::unique_ptr<HexagonSubtarget>> SubtargetMap;

llvm/lib/Target/LoongArch/LoongArchISelLowering.h

@@ -21,7 +21,6 @@
 
 namespace llvm {
 class LoongArchSubtarget;
-struct LoongArchRegisterInfo;
 namespace LoongArchISD {
 enum NodeType : unsigned {
   FIRST_NUMBER = ISD::BUILTIN_OP_END,

llvm/lib/Target/RISCV/RISCV.h

@@ -18,13 +18,8 @@
 #include "llvm/Target/TargetMachine.h"
 
 namespace llvm {
-class AsmPrinter;
 class FunctionPass;
 class InstructionSelector;
-class MCInst;
-class MCOperand;
-class MachineInstr;
-class MachineOperand;
 class PassRegistry;
 class RISCVRegisterBankInfo;
 class RISCVSubtarget;