arc: Remove mpy_dest_reg_operand predicate

The mpy_dest_reg_operand is just a wrapper for
register_operand. Remove it.

gcc/

	* config/arc/arc.md (mulsi3_700): Update pattern.
	(mulsi3_v2): Likewise.
	* config/arc/predicates.md (mpy_dest_reg_operand): Remove it.

Signed-off-by: Claudiu Zissulescu <claziss@gmail.com>