AVR: Add -mfuse-ifelse for better control of pass avr-ifelse.

This patch adds -m[no-]fuse-ifelse as new option, so that
the avr-ifelse pass can be deactivated.

gcc/
	* config/avr/avr.opt (-mfuse-ifelse): New option.
	* config/avr/avr-passes.cc (avr_pass_ifelse::gate): Don't
	return true when avropt_fuse_ifelse is false.
	* config/avr/avr.opt.urls (mfuse-ifelse): Add URL.
	* common/config/avr/avr-common.cc (avr_option_optimization_table)
	[OPT_LEVELS_1_PLUS]: Set OPT_mfuse_ifelse to 1.
	* doc/invoke.texi (AVR Options): Document it.