[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.