[X86] Don't emit COPY_TO_REG to ABCD registers before EXTRACT_SUBREG of sub_8bit_hi

I'm pretty sure that InstrEmitter::EmitSubregNode will take care of this itself by calling ConstrainForSubReg which in turn calls TRI->getSubClassWithSubReg.

I think Jakob Stoklund Olesen alluded to this in his commit message for r141207 which added the code to EmitSubregNode.

Differential Revision: https://reviews.llvm.org/D37843

llvm-svn: 313557