S12Z: GAS: Issue warning if TFR/EXG have identical source and destination.

S12Z: GAS: Issue warning if TFR/EXG have identical source and destination.

It is permissible for the source and destination operands of TFR and EXG to be
the same register.  However it is a pointless instruction and anyone writing it
has probably made a mistake.  This change emits a warning if such an instruction
is encountered.

gas/
	* config/tc-s12z.c (tfr): Emit warning if operands are the same.
	* testsuite/gas/s12z/exg.d: New test case.
	* testsuite/gas/s12z/exg.l: New file.