[X86] Don't emit X86::MOV8rr_NOREX from X86InstrInfo::copyPhysReg.

This hook is called after register allocation with two physical registers. We don't need a separate instruction at that time to force register class constraints. I left in the assert though. We also have a fatal error in X86MCCodeEmitter if we ever encode an H-reg and a REX prefix.

llvm-svn: 314248