AMDGPU: remove a couple of unused variables

AMDGPU: remove a couple of unused variables

	lib/Target/AMDGPU/SIRegisterInfo.cpp: In member function 'void llvm::SIRegisterInfo::spillSGPR(llvm::MachineBasicBlock::iterator, int, llvm::RegScavenger*) const':
	lib/Target/AMDGPU/SIRegisterInfo.cpp:572:30: warning: variable 'SubRC' set but not used [-Wunused-but-set-variable]
	   const TargetRegisterClass *SubRC = nullptr;
	                              ^
	lib/Target/AMDGPU/SIRegisterInfo.cpp: In member function 'void llvm::SIRegisterInfo::restoreSGPR(llvm::MachineBasicBlock::iterator, int, llvm::RegScavenger*) const':
	lib/Target/AMDGPU/SIRegisterInfo.cpp:723:30: warning: variable 'SubRC' set but not used [-Wunused-but-set-variable]
	   const TargetRegisterClass *SubRC = nullptr;
	                              ^

The variable was assigned to, but never used.  The functions called did not
mutate state.  Simplify the logic and remove the variable.  Identified by gcc
5.4.0.

llvm-svn: 288601