[X86][AsmParser] Don't allow X86::DX in CheckBaseRegAndIndexRegAndScale.

This was here because out and in instructions allow '(%dx)' even though its not a memory reference. To handle this we build a special operand for the DX register reference before we get to the call to CheckBaseRegAndIndexRegAndScale. So we no longer need this special case.

llvm-svn: 350483