[X86][AsmParser] Rework the in/out (%dx) hack one more time.

This patch adds a new token type specifically for (%dx). We will now always create this token when we parse (%dx). After all operands have been parsed, if the mnemonic is in/out we'll morph this token to a regular register token. Otherwise we keep it as the special DX token which won't match any instructions.

This removes the need for passing Mnemonic through the parsing functions. It also seems closer to gas where when its used on the wrong instruction it just gets diagnosed as an invalid operand rather than a bad memory address.

llvm-svn: 336218