VAX: Handle subtracting from self with QMATH DImode add/sub

VAX: Handle subtracting from self with QMATH DImode add/sub

Remove an assertion the failure of which has not been actually observed,
but which appears clearly dangerous, for when the QMATH DImode add/sub
handler is invoked with the subtrahend and the minuend both the same.
Instead handle the operation by emitting a move of constant 0 to the
output operand.  Adjust the relevant inline comment accordingly.

	gcc/
	* config/vax/vax.c (vax_expand_addsub_di_operands): Handle equal
	input operands with subtraction.