avr.md - Tweak xor insn constraints.

xor insn can handle some more values without the requirement of a
scratch register.  This patch adds a new constraint alternative for
such values.  The output function avr_out_bitop already handles
these cases, so no change is needed there.

gcc/
	* config/avr/constraints.md (CX2, CX3, CX4): New constraints.
	* config/avr/avr-protos.h (avr_xor_noclobber_dconst): New proto.
	* config/avr/avr.cc (avr_xor_noclobber_dconst): New function.
	* config/avr/avr.md (xorhi3, *xorhi3): Add "d,0,CX2,X" alternative.
	(xorpsi3, *xorpsi3): Add "d,0,CX3,X" alternative.
	(xorsi3, *xorsi3): Add "d,0,CX4,X" alternative.