[AMDGPU] Forward-declare TargetRegisterClass (NFC)

AMDGPUInstructionSelector.h needs TargetRegisterClass but relies on a
forward declaration of TargetRegisterClass in InstructionSelector.h.
This patch adds a forward declaration right in
AMDGPUInstructionSelector.h.

While we are at it, this patch removes the one in
InstructionSelector.h, where it is unnecessary.

Author: kazutakahirata

llvm/include/llvm/CodeGen/GlobalISel/InstructionSelector.h

@@ -39,7 +39,6 @@ class MachineOperand;
 class MachineRegisterInfo;
 class RegisterBankInfo;
 class TargetInstrInfo;
-class TargetRegisterClass;
 class TargetRegisterInfo;
 
 /// Container class for CodeGen predicate results.

llvm/lib/Target/AMDGPU/AMDGPUInstructionSelector.h

@@ -45,6 +45,7 @@ class RegisterBank;
 class SIInstrInfo;
 class SIMachineFunctionInfo;
 class SIRegisterInfo;
+class TargetRegisterClass;
 
 class AMDGPUInstructionSelector final : public InstructionSelector {
 private: