Small cleanup on x86-64: Use REG-IN-SIZE instead of MAKE-{BYTE,DWORD}-TN.

In VOP definitions, when a TN for a part of a register is needed,
in some places REG-IN-SIZE is used, in others MAKE-BYTE-TN or
MAKE-DWORD-TN. REG-IN-SIZE can do everything the other two functions
can, so use it in all cases to make the code more regular, and delete
the obsolete functions.